add_filter( string $hook_name, callable $callback, int $priority = 10, int $accepted_args = 1 ): true

User Contributed Notes

1. Skip to note 9 content

   Nagdy 8 years ago

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   Example: Let’s add extra sections to TwentySeventeen Front page.
   By default, TwentySeventeen theme has 4 sections for the front page. This example will make them 6

   add_filter( 'twentyseventeen_front_page_sections', 'prefix_custom_front_page_sections' );
   	
   function prefix_custom_front_page_sections( $num_sections )
   {
   		return 6;
   }
2. Skip to note 10 content

   mkormendy 5 years ago

   You must log in to vote on the helpfulness of this noteVote results for this note: 6You must log in to vote on the helpfulness of this note

   To pass a variable to the called function of the filter, you can use closures (since PHP 5.3+) when the argument is not available in the original coded apply_filters. For example:

   add_filter('wp_footer', function($arguments) use ($myvar) { 
       return $myvar;
   }, $priority_integer, $accepted_arguments_integer);
3. Skip to note 11 content

   Rinku Y 9 years ago

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   Example: Let display custom length of post excerpt.

   if( ! function_exists( 'prefix_custom_excerpt_length' ) )
   {
   	function prefix_custom_excerpt_length( $length )
   	{
   		return 40;
   	}
   }
   add_filter( 'excerpt_length', 'prefix_custom_excerpt_length', 999 );

   By default, WordPress display 57 character. you can set custom length using above code. this time except length will be 40. it is a nice and easiest use of add_filter.
4. Skip to note 12 content

   Xaib Aslam 5 years ago

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   Example: If you want to inject a CLASS/ID CSS in content. Let’s add extra CLASS/ID to post content.

   //Add Class/ID to Post Content
   add_filter('the_content', 'xai_my_class');
   function xai_my_class($content)
   {
       //Replace the instance with the Class/ID markup.
       $string = '<ul'; //your tag
       $replace = '<ul class="detail-list"'; //add your class/id and tag
       $content = str_replace( $string, $replace, $content );
       return $content;
   }
5. Skip to note 13 content

   biziclop 4 years ago

   You must log in to vote on the helpfulness of this noteVote results for this note: 1You must log in to vote on the helpfulness of this note

   In the special case you have to add a filter before WordPress starts, you can create and pre-populate the global $wp_filter array instead of using the not-yet available add_filter (or add_action) function:

   // Instead of add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ):
   $GLOBALS['wp_filter'][ $tag ][ $priority ][] = array(
     'function'      => $function_to_add,
     'accepted_args' => $accepted_args
   );

   WP_Hook::build_preinitialized_hooks will automatically take care of the rest.
   Though to remove your filter, you can only use the remove_all_filters() function.
6. Skip to note 14 content

   jon 5 years ago

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   The Callback $function_to_add need not be defined until the Filter hook fires. This means that:

   1. The add_filter function does not check that $function_to_add exists
   2. The function statement for $function_to_add can be defined after the add_filter statement, even in a conditional (e.g. – if block) where the $function_to_add function does not actually exist until after the add_filter function executes
   3. If the Filter hook never fires, an undefined $function_to_add function will not be reported as an error

   Point 3 needs to be considered during Testing or Quality Control

   An undefined $function_to_add function detected when a Filter hook fires is reported as a Warning error:
   Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'reg_public1' not found or invalid function name in /var/www/example.com/public_html/wp-includes/class-wp-hook.php on line 288
7. Skip to note 15 content

   Pilate Chinyengetere 9 months ago

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   If you output something inside the callback, the output location is unpredictable.
   Cause: the returned value is actually used somewhere else (more likely a bit far from where the function would be called). Therefore the output will go where the function was called while the return value is used somewhere else. I’m not sure when you would use this though :).

   add_filter( 'the_content', 'wpdocs_append_to_content', 9 );
   
   function wpdocs_append_to_content( $content ) {
           // output something
           echo 'I Don\'t Know Where I will end up being printed';
   
   	$additional_content = 'Appended to content';
   	$content = $content . $additional_content;
   
   	return $content;
   }
8. Skip to note 16 content

   Codex 10 years ago

   You must log in to vote on the helpfulness of this noteVote results for this note: -5You must log in to vote on the helpfulness of this note

   Example
   The filter img_caption_shortcode is applied in media.php using the following call:

   // Allow plugins/themes to override the default caption template.
   $output = apply_filters( 'img_caption_shortcode', '', $attr, $content );
   if ( $output != '' )
   	return $output;

   The target filter function will be called with three arguments:

   ” <= This is normally the value the filter will be modifying
   $attr
   $content

   In order for the filter function to actually receive the full argument list, the call to add_filter() must be modified to specify there are 3 arguments on the parameter list.

   add_filter('img_caption_shortcode', 'my_img_caption_shortcode_filter',10,3);
   
   /**
    * Filter to replace the  shortcode text with HTML5 compliant code
    *
    * @return text HTML content describing embedded figure
    **/
   function my_img_caption_shortcode_filter($val, $attr, $content = null)
   {
   	extract( shortcode_atts( array(
   		'id'	=> '',
   		'align'	=> '',
   		'width'	=> '',
   		'caption' => ''
   	), $attr ) );
   	
   	if ( 1 > (int) $width || empty($caption) )
   		return $val;
   
   	$capid = '';
   	if ( $id ) {
   		$id = esc_attr( $id );
   		$capid = 'id="figcaption_' . $id . '" ';
   		$id = 'id="' . $id . '" aria-labelledby="figcaption_' . $id . '" ';
   	}
   
   	return '<figure ' . $id . 'class="wp-caption ' . esc_attr($align) . '" style="width: '
   	. (10 + (int) $width) . 'px">' . do_shortcode( $content ) . '<figcaption ' . $capid 
   	. 'class="wp-caption-text">' . $caption . '</figcaption></figure>';
   }