wp_kses_attr( string $element, string $attr, array[]|string $allowed_html, string[] $allowed_protocols )

Removes all attributes, if none are allowed for this element.


Description

If some are allowed it calls wp_kses_hair() to split them further, and then it builds up new HTML code from the data that kses_hair() returns. It also removes < and > characters, if there are any left. One more thing it does is to check if the tag has a closing XHTML slash, and if it does, it puts one in the returned code as well.

An array of allowed values can be defined for attributes. If the attribute value doesn’t fall into the list, the attribute will be removed from the tag.

Attributes can be marked as required. If a required attribute is not present, KSES will remove all attributes from the tag. As KSES doesn’t match opening and closing tags, it’s not possible to safely remove the tag itself, the safest fallback is to strip all attributes from the tag, instead.


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Parameters

$element

(string) (Required) HTML element/tag.

$attr

(string) (Required) HTML attributes from HTML element to closing HTML element tag.

$allowed_html

(array[]|string) (Required) An array of allowed HTML elements and attributes, or a context name such as 'post'. See wp_kses_allowed_html() for the list of accepted context names.

$allowed_protocols

(string[]) (Required) Array of allowed URL protocols.


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Return

(string) Sanitized HTML element.


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Source

File: wp-includes/kses.php

function wp_kses_attr( $element, $attr, $allowed_html, $allowed_protocols ) {
	if ( ! is_array( $allowed_html ) ) {
		$allowed_html = wp_kses_allowed_html( $allowed_html );
	}

	// Is there a closing XHTML slash at the end of the attributes?
	$xhtml_slash = '';
	if ( preg_match( '%\s*/\s*$%', $attr ) ) {
		$xhtml_slash = ' /';
	}

	// Are any attributes allowed at all for this element?
	$element_low = strtolower( $element );
	if ( empty( $allowed_html[ $element_low ] ) || true === $allowed_html[ $element_low ] ) {
		return "<$element$xhtml_slash>";
	}

	// Split it.
	$attrarr = wp_kses_hair( $attr, $allowed_protocols );

	// Check if there are attributes that are required.
	$required_attrs = array_filter(
		$allowed_html[ $element_low ],
		function( $required_attr_limits ) {
			return isset( $required_attr_limits['required'] ) && true === $required_attr_limits['required'];
		}
	);

	/*
	 * If a required attribute check fails, we can return nothing for a self-closing tag,
	 * but for a non-self-closing tag the best option is to return the element with attributes,
	 * as KSES doesn't handle matching the relevant closing tag.
	 */
	$stripped_tag = '';
	if ( empty( $xhtml_slash ) ) {
		$stripped_tag = "<$element>";
	}

	// Go through $attrarr, and save the allowed attributes for this element in $attr2.
	$attr2 = '';
	foreach ( $attrarr as $arreach ) {
		// Check if this attribute is required.
		$required = isset( $required_attrs[ strtolower( $arreach['name'] ) ] );

		if ( wp_kses_attr_check( $arreach['name'], $arreach['value'], $arreach['whole'], $arreach['vless'], $element, $allowed_html ) ) {
			$attr2 .= ' ' . $arreach['whole'];

			// If this was a required attribute, we can mark it as found.
			if ( $required ) {
				unset( $required_attrs[ strtolower( $arreach['name'] ) ] );
			}
		} elseif ( $required ) {
			// This attribute was required, but didn't pass the check. The entire tag is not allowed.
			return $stripped_tag;
		}
	}

	// If some required attributes weren't set, the entire tag is not allowed.
	if ( ! empty( $required_attrs ) ) {
		return $stripped_tag;
	}

	// Remove any "<" or ">" characters.
	$attr2 = preg_replace( '/[<>]/', '', $attr2 );

	return "<$element$attr2$xhtml_slash>";
}


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Changelog

Changelog
Version Description
5.9.0 Added support for an array of allowed values for attributes. Added support for required attributes.
1.0.0 Introduced.

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