wp_kses_attr

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

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 `wp_kses_hair()` returns. It also
removes “ 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.

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>";
}