PHP Coding Standards Edit

These PHP coding standards are intended for the WordPress community as a whole. They are mandatory for WordPress Core and we encourage you to use them for your themes and plugins as well.

While themes and plugins may choose to follow a different coding style, these coding standards are not just about code style, but also encompass established best practices regarding interoperability, translatability, and security in the WordPress ecosystem, so, even when using a different code style, we recommend you still adhere to the WordPress Coding Standards in regard to these best practices.

While not all code may fully comply with these standards (yet), all newly committed and/or updated code should fully comply with these coding standards.

Also see the PHP Inline Documentation Standards for further guidelines.

If you want to automatically check your code against this standard, you can use the official WordPress Coding Standards tooling, which is run using PHP_CodeSniffer.

General

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Opening and Closing PHP Tags

When embedding multi-line PHP snippets within an HTML block, the PHP open and close tags must be on a line by themselves.

Correct (Multiline):

function foo() {
    ?>
    <div>
        <?php
        echo esc_html(
            bar(
                $baz,
                $bat
            )
        );
        ?>
    </div>
    <?php
}

Correct (Single Line):

<input name="<?php echo esc_attr( $name ); ?>" />

Incorrect:

if ( $a === $b ) { ?>
<some html>
<?php }

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No Shorthand PHP Tags

Important: Never use shorthand PHP start tags. Always use full PHP tags.

Correct:

<?php ... ?>
<?php echo esc_html( $var ); ?>

Incorrect:

<? ... ?>
<?= esc_html( $var ) ?>

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Single and Double Quotes

Use single and double quotes when appropriate. If you’re not evaluating anything in the string, use single quotes. You should almost never have to escape quotes in a string, because you can just alternate your quoting style, like so:

echo '<a href="/static/link" class="button button-primary">Link name</a>';
echo "<a href='{$escaped_link}'>text with a ' single quote</a>";

Text that goes into HTML or XML attributes should be escaped so that single or double quotes do not end the attribute value and invalidate the HTML, causing a security issue. See Data Validation in the Plugin Handbook for further details.

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Writing include/require statements

Because include[_once] and require[_once] are language constructs, they do not need parentheses around the path, so those shouldn’t be used. There should only be one space between the path and the include/require keywords.

It is strongly recommended to use require[_once] for unconditional includes. When using include[_once], PHP will throw a warning when the file is not found but will continue execution, which will almost certainly lead to other errors/warnings/notices being thrown if your application depends on the file loaded, potentially leading to security leaks. For that reason, require[_once] is generally the better choice as it will throw a Fatal Error if the file cannot be found.

// Correct.
require_once ABSPATH . 'file-name.php';

// Incorrect.
include_once  ( ABSPATH . 'file-name.php' );
require_once     __DIR__ . '/file-name.php';

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Naming

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Naming Conventions

Use lowercase letters in variable, action/filter, and function names (never camelCase). Separate words via underscores. Don’t abbreviate variable names unnecessarily; let the code be unambiguous and self-documenting.

function some_name( $some_variable ) {}

For function parameter names, it is strongly recommended to avoid reserved keywords as names, as it leads to hard to read and confusing code when using the PHP 8.0 “named parameters in function calls” feature.
Also keep in mind that renaming a function parameter should be considered a breaking change since PHP 8.0, so name function parameters with due care!

Class, trait, interface and enum names should use capitalized words separated by underscores. Any acronyms should be all upper case.

class Walker_Category extends Walker {}
class WP_HTTP {}

interface Mailer_Interface {}
trait Forbid_Dynamic_Properties {}
enum Post_Status {}

Constants should be in all upper-case with underscores separating words:

define( 'DOING_AJAX', true );

Files should be named descriptively using lowercase letters. Hyphens should separate words.

my-plugin-name.php

Class file names should be based on the class name with class- prepended and the underscores in the class name replaced with hyphens, for example, WP_Error becomes:

class-wp-error.php

This file-naming standard is for all current and new files with classes. There is one exception to this rule for three legacy files: class.wp-dependencies.php, class.wp-scripts.php, class.wp-styles.php. Those files are prepended with class., a dot after the word class instead of a hyphen.

Files containing template tags in the wp-includes directory should have -template appended to the end of the name so that they are obvious.

general-template.php

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Interpolation for Naming Dynamic Hooks

Dynamic hooks should be named using interpolation rather than concatenation for readability and discoverability purposes.

Dynamic hooks are hooks that include dynamic values in their tag name, e.g. {$new_status}_{$post->post_type} (publish_post).

Variables used in hook tags should be wrapped in curly braces { and }, with the complete outer tag name wrapped in double quotes. This is to ensure PHP can correctly parse the given variables’ types within the interpolated string.

do_action( "{$new_status}_{$post->post_type}", $post->ID, $post );

Where possible, dynamic values in tag names should also be as succinct and to the point as possible. $user_id is much more self-documenting than, say, $this->id.

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Whitespace

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Space Usage

Always put spaces after commas, and on both sides of logical, arithmetic, comparison, string and assignment operators.

SOME_CONST === 23;
foo() && bar();
! $foo;
array( 1, 2, 3 );
$baz . '-5';
$term .= 'X';
if ( $object instanceof Post_Type_Interface ) {}
$result = 2 ** 3; // 8.

Put spaces on both sides of the opening and closing parentheses of control structure blocks.

foreach ( $foo as $bar ) { ...

When defining a function, do it like so:

function my_function( $param1 = 'foo', $param2 = 'bar' ) { ...

function my_other_function() { ...

When calling a function, do it like so:

my_function( $param1, func_param( $param2 ) );
my_other_function();

When performing logical comparisons, do it like so:

if ( ! $foo ) { ...

Type casts must be lowercase. Always prefer the short form of type casts, (int) instead of (integer) and (bool) rather than (boolean). For float casts use (float), not (real) which is deprecated in PHP 7.4, and removed in PHP 8:

foreach ( (array) $foo as $bar ) { ...

$foo = (bool) $bar;

When referring to array items, only include a space around the index if it is a variable, for example:

$x = $foo['bar']; // Correct.
$x = $foo[ 'bar' ]; // Incorrect.

$x = $foo[0]; // Correct.
$x = $foo[ 0 ]; // Incorrect.

$x = $foo[ $bar ]; // Correct.
$x = $foo[$bar]; // Incorrect.

In a switch block, there must be no space between the case condition and the colon.

switch ( $foo ) {
    case 'bar': // Correct.
    case 'ba' : // Incorrect.
}

Unless otherwise specified, parentheses should have spaces inside them.

if ( $foo && ( $bar || $baz ) ) { ...

my_function( ( $x - 1 ) * 5, $y );

When using increment (++) or decrement (--) operators, there should be no spaces between the operator and the variable it applies to.

// Correct.
for ( $i = 0; $i < 10; $i++ ) {}

// Incorrect.
for ( $i = 0; $i < 10; $i ++ ) {}
++   $b; // Multiple spaces.

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Indentation

Your indentation should always reflect logical structure. Use real tabs, not spaces, as this allows the most flexibility across clients.

Exception: if you have a block of code that would be more readable if things are aligned, use spaces:

[tab]$foo   = 'somevalue';
[tab]$foo2  = 'somevalue2';
[tab]$foo34 = 'somevalue3';
[tab]$foo5  = 'somevalue4';

For associative arrays, each item should start on a new line when the array contains more than one item:

$query = new WP_Query( array( 'ID' => 123 ) );
$args = array(
[tab]'post_type'   => 'page',
[tab]'post_author' => 123,
[tab]'post_status' => 'publish',
);

$query = new WP_Query( $args );

Note the comma after the last array item: this is recommended because it makes it easier to change the order of the array, and makes for cleaner diffs when new items are added.

$my_array = array(
[tab]'foo'   => 'somevalue',
[tab]'foo2'  => 'somevalue2',
[tab]'foo3'  => 'somevalue3',
[tab]'foo34' => 'somevalue3',
);

For switch control structures, case statements should be indented one tab from the switch statement and the contents of the case should be indented one tab from the case condition statement.

switch ( $type ) {
[tab]case 'foo':
[tab][tab]some_function();
[tab][tab]break;
[tab]case 'bar':
[tab][tab]some_function();
[tab][tab]break;
}

Rule of thumb: Tabs should be used at the beginning of the line for indentation, while spaces can be used mid-line for alignment.

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Remove Trailing Spaces

Remove trailing whitespace at the end of each line. Omitting the closing PHP tag at the end of a file is preferred. If you use the tag, make sure you remove trailing whitespace.

There should be no trailing blank lines at the end of a function body.

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Formatting

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Brace Style

Braces shall be used for all blocks in the style shown here:

if ( condition ) {
    action1();
    action2();
} elseif ( condition2 && condition3 ) {
    action3();
    action4();
} else {
    defaultaction();
}

If you have a really long block, consider whether it can be broken into two or more shorter blocks, functions, or methods, to reduce complexity, improve ease of testing, and increase readability.

Braces should always be used, even when they are not required:

if ( condition ) {
    action0();
}

if ( condition ) {
    action1();
} elseif ( condition2 ) {
    action2a();
    action2b();
}

foreach ( $items as $item ) {
    process_item( $item );
}

Note that requiring the use of braces means that single-statement inline control structures are prohibited. You are free to use the alternative syntax for control structures (e.g. if/endif, while/endwhile)—especially in templates where PHP code is embedded within HTML, for instance:

<?php if ( have_posts() ) : ?>
    <div class="hfeed">
        <?php while ( have_posts() ) : the_post(); ?>
            <article id="<?php echo esc_attr( 'post-' . get_the_ID() ); ?>" class="<?php echo esc_attr( get_post_class() ); ?>">
                <!-- ... -->
            </article>
        <?php endwhile; ?>
    </div>
<?php endif; ?>

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Declaring Arrays

Using long array syntax ( array( 1, 2, 3 ) ) for declaring arrays is generally more readable than short array syntax ( [ 1, 2, 3 ] ), particularly for those with vision difficulties. Additionally, it’s much more descriptive for beginners.

Arrays must be declared using long array syntax.

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Multiline Function Calls

When splitting a function call over multiple lines, each parameter must be on a separate line. Single line inline comments can take up their own line.

Each parameter must take up no more than a single line. Multi-line parameter values must be assigned to a variable and then that variable should be passed to the function call.

$bar = array(
    'use_this' => true,
    'meta_key' => 'field_name',
);
$baz = sprintf(
    /* translators: %s: Friend's name */
    __( 'Hello, %s!', 'yourtextdomain' ),
    $friend_name
);

$a = foo(
    $bar,
    $baz,
    /* translators: %s: cat */
    sprintf( __( 'The best pet is a %s.' ), 'cat' )
);

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Type declarations

Type declarations must have exactly one space before and after the type. The nullability operator (?) is regarded as part of the type declaration and there should be no space between this operator and the actual type. Class/interface/enum name based type declarations should use the case of the class/interface/enum name as declared, while the keyword-based type declarations should be lowercased.

Return type declarations should have no space between the closing parenthesis of the function declaration and the colon starting a return type.

These rules apply to all structures allowing for type declarations: functions, closures, enums, catch conditions as well as the PHP 7.4 arrow functions and typed properties.

// Correct.
function foo( Class_Name $parameter, callable $callable, int $number_of_things = 0 ) {
    // Do something.
}

function bar(
    Interface_Name&Concrete_Class $param_a,
    string|int $param_b,
    callable $param_c = 'default_callable'
): User|false {
    // Do something.
}

// Incorrect.
function baz(Class_Name $param_a, String$param_b,      CALLABLE     $param_c )   :   ?   iterable   {
    // Do something.
}

Note:
Type declaration usage has the following restrictions based on the minimum required PHP version of an application, whether it is WordPress Core, a plugin or a theme:

  • The scalar bool, int, float, and string type declarations cannot be used until the minimum PHP version is PHP 7.0 or higher.
  • Return type declarations cannot be used until the minimum PHP version is PHP 7.0 or higher.
  • Nullable type declarations cannot be used until the minimum PHP version is PHP 7.1 or higher.
  • The iterable and void type declarations cannot be used until the minimum PHP version is PHP 7.1 or higher. The void type can only be used as a return type.
  • The object type declaration cannot be used until the minimum PHP version is PHP 7.2 or higher.
  • Property type declarations cannot be used until the minimum PHP version is PHP 7.4 or higher.
  • static (return type only) cannot be used until the minimum PHP version is PHP 8.0 or higher.
  • mixed type cannot be used until the minimum PHP version is PHP 8.0 or higher. Take note that the mixed type includes null, so cannot be made nullable.
  • Union types cannot be used until the minimum PHP version is PHP 8.0 or higher.
  • Intersection types cannot be used until the minimum PHP version is PHP 8.1 or higher.
  • never (return type only) cannot be used until the minimum PHP version is PHP 8.1 or higher.
  • Disjunctive normal form types (combining union types and intersection types) cannot be used until the minimum PHP version is PHP 8.2 or higher.

Usage in WordPress Core

Warning:
Adding type declarations to existing WordPress Core functions should be done with extreme care.

The function signature of any function (method) which can be overloaded by plugins or themes should not be touched.
This leaves, for now, only unconditionally declared functions in the global namespace, private class methods, and code new to Core, as candidates for adding type declarations.

Note: Using the array keyword in type declarations is strongly discouraged for now, as most often, it would be better to use iterable to allow for more flexibility in the implementation and that keyword is not yet available for use in WordPress Core until the minimum requirements are raised to PHP 7.1.

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Magic constants

The PHP native __*__ magic constants, like __CLASS__ and __DIR__, should be written in uppercase when used.

When using the ::class constant for class name resolution, the class keyword should be in lowercase and there should be no spaces around the :: operator.

// Correct.
add_action( 'action_name', array( __CLASS__, 'method_name' ) );
add_action( 'action_name', array( My_Class::class, 'method_name' ) );

// Incorrect.
require_once __dIr__ . '/relative-path/file-name.php';
add_action( 'action_name', array( My_Class :: CLASS, 'method_name' ) );

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Spread operator ...

When using the spread operator, there should be one space or a new line with the appropriate indentation before the spread operator. There should be no spaces between the spread operator and the variable/function call it applies to. When combining the spread operator with the reference operator (&), there should be no spaces between them.

// Correct.
function foo( &...$spread ) {
    bar( ...$spread );

    bar(
        array( ...$foo ),
        ...array_values( $keyed_array )
    );
}

// Incorrect.
function fool( &   ... $spread ) {
    bar(...
             $spread );

    bar(
        [...  $foo ],... array_values( $keyed_array )
    );
}

Note:
The spread operator (or splat operator as it’s known in other languages) can be used for packing arguments in function declarations (variadic functions) and unpacking them in function calls as of PHP 5.6. Since PHP 7.4, the spread operator is also used for unpacking numerically-indexed arrays, with string-keyed array unpacking available since PHP 8.1.
When used in a function declaration, the spread operator can only be used with the last parameter.

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Declare Statements, Namespace, and Import Statements

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Namespace declarations

Each part of a namespace name should consist of capitalized words separated by underscores.

Namespace declarations should have exactly one blank line before the declaration and at least one blank line after.

namespace Prefix\Admin\Domain_URL\Sub_Domain\Event; // Correct.

There should be only one namespace declaration per file, and it should be at the top of the file. Namespace declarations using curly brace syntax are not allowed. Explicit global namespace declaration (namespace declaration without name) are also not allowed.

// Incorrect: namespace declaration using curly brace syntax.
namespace Foo {
    // Code.
}

// Incorrect: namespace declaration for the global namespace.
namespace {
    // Code.
}

There is currently no timeline for introducing namespaces to WordPress Core.

The use of namespaces in plugins and themes is strongly encouraged. It is a great way to prefix a lot of your code to prevent naming conflicts with other plugins, themes and/or WordPress Core.

Please do make sure you use a unique and long enough namespace prefix to actually prevent conflicts. Generally speaking, using a namespace prefix along the lines of Vendor\Project_Name is a good idea.

Warning:
The wp and WordPress namespace prefixes are reserved for WordPress itself.

Note:
Namespacing has no effect on variables, constants declared with define() or non-PHP native constructs, like the hook names as used in WordPress.
Those still need to be prefixed individually.

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Using import use statements

Using import use statements allows you to refer to constants, functions, classes, interfaces, namespaces, enums and traits that live outside of the current namespace.

Import use statements should be at the top of the file and follow the (optional) namespace declaration. They should follow a specific order based on the type of the import:

  1. use statements for namespaces, classes, interfaces, traits and enums
  2. use statements for functions
  3. use statements for constants

Aliases can be used to prevent name collisions (two classes in different namespaces using the same class name).
When using aliases, make sure the aliases follow the WordPress naming convention and are unique.

The following examples showcase the correct and incorrect usage of import use statements regarding things like spacing, groupings, leading backslashes, etc.

Correct:

namespace Project_Name\Feature;

use Project_Name\Sub_Feature\Class_A;
use Project_Name\Sub_Feature\Class_C as Aliased_Class_C;
use Project_Name\Sub_Feature\{
    Class_D,
    Class_E as Aliased_Class_E,
}

use function Project_Name\Sub_Feature\function_a;
use function Project_Name\Sub_Feature\function_b as aliased_function;

use const Project_Name\Sub_Feature\CONSTANT_A;
use const Project_Name\Sub_Feature\CONSTANT_D as ALIASED_CONSTANT;

// Rest of the code.

Incorrect:

namespace Project_Name\Feature;

use   const   Project_Name\Sub_Feature\CONSTANT_A; // Superfluous whitespace after the "use" and the "const" keywords.
use function Project_Name\Sub_Feature\function_a; // Function import after constant import. 
use \Project_Name\Sub_Feature\Class_C as aliased_class_c; // Leading backslash shouldn't be used, alias doesn't comply with naming conventions.
use Project_Name\Sub_Feature\{Class_D, Class_E   as   Aliased_Class_E} // Extra spaces around the "as" keyword, incorrect whitespace use inside the brace opener and closer.
use Vendor\Package\{ function function_a, function function_b,
     Class_C,
        const CONSTANT_NAME}; // Combining different types of imports in one use statement, incorrect whitespace use within group use statement.

class Foo {
    // Code.
}

use const \Project_Name\Sub_Feature\CONSTANT_D as Aliased_constant; // Import after class definition, leading backslash, naming conventions violation.
use function Project_Name\Sub_Feature\function_b as Aliased_Function; // Import after class definition, naming conventions violation.

// Rest of the code.

Alert:
Import use statements have no effect on dynamic class, function or constant names.
Group use statements are available from PHP 7.0, and trailing commas in group use statements are available from PHP 7.2.

Note:
Note that, unless you have implemented autoloading, the use statement won’t automatically load whatever is being imported. You’ll either need to set up autoloading or load the file containing the class/function/constant using a require/import statement, for the aliased constructs to be loaded when used.

Note about WordPres Core usage

While import use statements can already be used in WordPress Core, it is, for the moment, strongly discouraged.

Import use statements are most useful when combined with namespaces and a class autoloading implementation.
As neither of these are currently in place for WordPress Core and discussions about this are ongoing, holding off on adding import use statements to WordPress Core is the sensible choice for now.

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Object-Oriented Programming

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Only One Object Structure (Class/Interface/Trait) per File

For instance, if we have a file called class-example-class.php it can only contain one class in that file.

// Incorrect: file class-example-class.php.
class Example_Class {}

class Example_Class_Extended {}

The second class should be in its own file called class-example-class-extended.php.

// Correct: file class-example-class.php.
class Example_Class {}
// Correct: file class-example-class-extended.php.
class Example_Class_Extended {}

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Trait Use Statements

Trait use statements should be at the top of a class and should have exactly one blank line before the first use statement, and at least one blank line after the last statement. The only exception is when the class only contains trait use statements, in which case the blank line after may be omitted.

The following code examples show the formatting requirements for trait use statements regarding things like spacing, grouping and indentation.

// Correct.
class Foo {

    use Bar_Trait;
    use Foo_Trait,
        Bazinga_Trait {
        Bar_Trait::method_name insteadof Bar_Trait;
        Bazinga_Trait::method_name as bazinga_method;
    }
    use Loopy_Trait { eat as protected; }

    public $baz = true;

    ...
}

// Incorrect.
class Foo {
    // No blank line before trait use statement, multiple spaces after the use keyword.
    use       Bar_Trait;

    /*
     * Multiple spaces when importing traits, no new line after opening brace.
     * Aliasing should be done on the same line as the method it's replacing.
     */
    use Foo_Trait,   Bazinga_Trait{Bar_Trait::method_name    insteadof     Foo_Trait; Bazinga_Trait::method_name
        as     bazinga_method;
            }; // Wrongly indented brace.
    public $baz = true; // Missing blank line after trait import.

    ...
}

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Visibility should always be declared

For all constructs that allow it (properties, methods, class constants since PHP 7.1), visibility should be explicitly declared.
Using the var keyword for property declarations is not allowed.

// Correct.
class Foo {
    public $foo;

    protected function bar() {}
}

// Incorrect.
class Foo {
    var $foo;

    function bar() {}
}

Usage in WordPress Core

Visibility for class constants can not be used in WordPress Core until the minimum PHP version has been raised to PHP 7.1.

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Visibility and modifier order

When using multiple modifiers for a class declaration, the order should be as follows:

  1. First the optional abstract or final modifier.
  2. Next, the optional readonly modifier.

When using multiple modifiers for a constant declaration inside object-oriented structures, the order should be as follows:

  1. First the optional final modifier.
  2. Next, the visibility modifier.

When using multiple modifiers for a property declaration, the order should be as follows:

  1. First a visibility modifier.
  2. Next, the optional static or readonly modifier (these keywords are mutually exclusive).
  3. Finally, the optional type declaration.

When using multiple modifiers for a method declaration, the order should be as follows:

  1. First the optional abstract or final modifier.
  2. Then, a visibility modifier.
  3. Finally, the optional static modifier.
// Correct.
abstract readonly class Foo {
    private const LABEL = 'Book';

    public static $foo;

    private readonly string $bar;

    abstract protected static function bar();
}

// Incorrect.
class Foo {
    protected final const SLUG = 'book';

    static public $foo;

    static protected final function bar() {
        // Code.
    };
}

Note:
– Visibility for OO constants can be declared since PHP 7.1.
– Typed properties are available since PHP 7.4.
– Readonly properties are available since PHP 8.1.
final modifier for constants in object-oriented structures is available since PHP 8.1.
– Readonly classes are available since PHP 8.2.

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Object Instantiation

When instantiating a new object instance, parenthesis must always be used, even when not strictly necessary.
There should be no space between the name of the class being instantiated and the opening parenthesis.

// Correct.
$foo = new Foo();
$anonymous_class = new class( $parameter ) { ... };
$instance = new static();

// Incorrect.
$foo = new Foo;
$anonymous_class = new class ( $input ) { ... };

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Control Structures

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Use elseif, not else if

else if is not compatible with the colon syntax for if|elseif blocks. For this reason, use elseif for conditionals.

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Yoda Conditions

if ( true === $the_force ) {
    $victorious = you_will( $be );
}

When doing logical comparisons involving variables, always put the variable on the right side and put constants, literals, or function calls on the left side. If neither side is a variable, the order is not important. (In computer science terms, in comparisons always try to put l-values on the right and r-values on the left.)

In the above example, if you omit an equals sign (admit it, it happens even to the most seasoned of us), you’ll get a parse error, because you can’t assign to a constant like true. If the statement were the other way around ( $the_force = true ), the assignment would be perfectly valid, returning 1, causing the if statement to evaluate to true, and you could be chasing that bug for a while.

A little bizarre, it is, to read. Get used to it, you will.

This applies to ==, !=, ===, and !==. Yoda conditions for <, >, <= or >= are significantly more difficult to read and are best avoided.

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Operators

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Ternary Operator

Ternary operators are fine, but always have them test if the statement is true, not false. Otherwise, it just gets confusing. (An exception would be using ! empty(), as testing for false here is generally more intuitive.)

The short ternary operator must not be used.

For example:

// (if statement is true) ? (do this) : (else, do this);
$musictype = ( 'jazz' === $music ) ? 'cool' : 'blah';
// (if field is not empty ) ? (do this) : (else, do this);

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Error Control Operator @

As noted in the PHP docs:

PHP supports one error control operator: the at sign (@). When prepended to an expression in PHP, any diagnostic error that might be generated by that expression will be suppressed.

While this operator does exist in Core, it is often used lazily instead of doing proper error checking. Its use is highly discouraged, as even the PHP docs also state:

Warning: Prior to PHP 8.0.0, it was possible for the @ operator to disable critical errors that will terminate script execution. For example, prepending @ to a call of a function that did not exist, by being unavailable or mistyped, would cause the script to terminate with no indication as to why.

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Increment/decrement operators

Pre-increment/decrement should be favoured over post-increment/decrement for stand-alone statements.

Pre-increment/decrement will increment/decrement and then return, while post-increment/decrement will return and then increment/decrement.
Using the “pre” version is slightly more performant and can prevent future bugs when code gets moved around.

// Correct: Pre-decrement.
--$a;

// Incorrect: Post-decrement.
$a--;

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Database

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Database Queries

Avoid touching the database directly. If there is a defined function that can get the data you need, use it. Database abstraction (using functions instead of queries) helps keep your code forward-compatible and, in cases where results are cached in memory, it can be many times faster.

If you must touch the database, consider creating a Trac ticket. There you can discuss the possibility of adding a new function to cover the functionality you wanted, for a future version of WordPress.

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Formatting SQL statements

When formatting SQL statements you may break them into several lines and indent if it is sufficiently complex to warrant it. Most statements work well as one line though. Always capitalize the SQL parts of the statement like UPDATE or WHERE.

Functions that update the database should expect their parameters to lack SQL slash escaping when passed. Escaping should be done as close to the time of the query as possible, preferably by using $wpdb->prepare()

$wpdb->prepare() is a method that handles escaping, quoting, and int-casting for SQL queries. It uses a subset of the sprintf() style of formatting. Example :

$var = "dangerous'"; // Raw data that may or may not need to be escaped.
$id = some_foo_number(); // Data we expect to be an integer, but we're not certain.

$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->posts SET post_title = %s WHERE ID = %d", $var, $id ) );

%s is used for string placeholders and %d is used for integer placeholders. Note that they are not ‘quoted’! $wpdb->prepare() will take care of escaping and quoting for us. The benefit of this is that it is easy to see at a glance whether something has been escaped or not because it happens right when the query happens.

See Data Validation in the Plugin Handbook for further details.

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Recommendations

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Self-Explanatory Flag Values for Function Arguments

Prefer string values to just true and false when calling functions.

// Incorrect.
function eat( $what, $slowly = true ) {
...
}
eat( 'mushrooms' );
eat( 'mushrooms', true ); // What does true mean?
eat( 'dogfood', false ); // What does false mean? The opposite of true?

PHP only supports named arguments as of PHP 8.0. However, as WordPress currently still supports older PHP versions, we cannot yet use those.
Without named arguments, the values of the flags are meaningless, and each time we come across a function call like the examples above, we have to search for the function definition. The code can be made more readable by using descriptive string values, instead of booleans.

// Correct.
function eat( $what, $speed = 'slowly' ) {
...
}
eat( 'mushrooms' );
eat( 'mushrooms', 'slowly' );
eat( 'dogfood', 'quickly' );

When more words are needed to describe the function parameters, an $args array may be a better pattern.

function eat( $what, $args ) {
...
}
eat ( 'noodles', array( 'speed' => 'moderate' ) );

Be careful when using this pattern, as it can lead to “Undefined array index” notices if input isn’t validated before use. Use this pattern only where it makes sense (i.e. multiple possible arguments), not just for the sake of it.

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Clever Code

In general, readability is more important than cleverness or brevity.

isset( $var ) || $var = some_function();

Although the above line is clever, it takes a while to grok if you’re not familiar with it. So, just write it like this:

if ( ! isset( $var ) ) {
    $var = some_function();
}

Unless absolutely necessary, loose comparisons should not be used, as their behaviour can be misleading.

Correct:

if ( 0 === strpos( $text, 'WordPress' ) ) {
    echo esc_html__( 'Yay WordPress!', 'textdomain' );
}

Incorrect:

if ( 0 == strpos( 'WordPress', 'foo' ) ) {
    echo esc_html__( 'Yay WordPress!', 'textdomain' );
}

Assignments must not be placed in conditionals.

Correct:

$data = $wpdb->get_var( '...' );
if ( $data ) {
    // Use $data.
}

Incorrect:

if ( $data = $wpdb->get_var( '...' ) ) {
    // Use $data.
}

In a switch statement, it’s okay to have multiple empty cases fall through to a common block. If a case contains a block, then falls through to the next block, however, this must be explicitly commented.

switch ( $foo ) {
    case 'bar':                // Correct, an empty case can fall through without comment.
    case 'baz':
        echo esc_html( $foo ); // Incorrect, a case with a block must break, return, or have a comment.
    case 'cat':
        echo 'mouse';
        break;                 // Correct, a case with a break does not require a comment.
    case 'dog':
        echo 'horse';
        // no break            // Correct, a case can have a comment to explicitly mention the fall through.
    case 'fish':
        echo 'bird';
        break;
}

The goto statement must never be used.

The eval() construct is very dangerous and is impossible to secure. Additionally, the create_function() function, which internally performs an eval(), is deprecated since PHP 7.2 and has been removed in PHP 8.0. Neither of these must be used.

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Closures (Anonymous Functions)

Where appropriate, closures may be used as an alternative to creating new functions to pass as callbacks. For example:

$caption = preg_replace_callback(
    '/<[a-zA-Z0-9]+(?: [^<>]+>)*/',
    function ( $matches ) {
        return preg_replace( '/[\r\n\t]+/', ' ', $matches[0] );
    },
    $caption
);

Closures should not be passed as filter or action callbacks, as removing these via remove_action() / remove_filter() is complex (at this time) (see #46635 for a proposal to address this).

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Regular Expressions

Perl compatible regular expressions (PCRE, preg_ functions) should be used in preference to their POSIX counterparts. Never use the /e switch, use preg_replace_callback instead.

It’s most convenient to use single-quoted strings for regular expressions since, contrary to double-quoted strings, they have only two metasequences which need escaping: \' and \\.

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Don’t extract()

Per #22400:

extract() is a terrible function that makes code harder to debug and harder to understand. We should discourage it’s [sic] use and remove all of our uses of it.

Joseph Scott has a good write-up of why it’s bad.

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Shell commands

Use of the backtick operator is not allowed.

Use of the backtick operator is identical to shell_exec(), and most hosts disable this function in the php.ini file for security reasons.