mustardBees/functions.php

## functions.php
<?php
/**
 * Add a class to tags.
 *
 * @param string $html The HTML content.
 * @param array $tags List of HTML tags to add a class to.
 * @param string $class The class to add.
 * @param bool $exception Optional. If the exception value has been defined, it'll take precedence.
 *
 * @return array|string|string[]|null
 */
function kanuka_add_class_to_tags( $html, $tags, $class, $exception = false ) {
	if ( strlen( trim( $html ) ) <= 0 ) {
		return;
	}

	libxml_use_internal_errors( true );
	$dom  = new DOMDocument( '1.0', 'utf-8' );
	$head = '<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head>';
	$dom->loadHTML( $head . $html );
	foreach ( $tags as $tag ) {
		$elements = $dom->getElementsByTagName( $tag );
		foreach ( $elements as $element ) {

			// Bail if we already have an explicitly defined class value.
			if ( $exception && false !== strpos( $element->getAttribute( 'class' ), $exception ) ) {
				continue;
			}

			$element->setAttribute( 'class', trim( $element->getAttribute( 'class' ) . ' ' . $class ) );
		}
	}
	$content = preg_replace( '/<\/?(!doctype|html|head|meta|body)[^>]*>/im', '', $dom->saveHTML() );
	libxml_use_internal_errors( false );

	return $content;
}
	<?php
	/**
	* Add a class to tags.
	*
	* @param string $html The HTML content.
	* @param array $tags List of HTML tags to add a class to.
	* @param string $class The class to add.
	* @param bool $exception Optional. If the exception value has been defined, it'll take precedence.
	*
	* @return array\|string\|string[]\|null
	*/
	function kanuka_add_class_to_tags( $html, $tags, $class, $exception = false ) {
	if ( strlen( trim( $html ) ) <= 0 ) {
	return;
	}

	libxml_use_internal_errors( true );
	$dom = new DOMDocument( '1.0', 'utf-8' );
	$head = '<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head>';
	$dom->loadHTML( $head . $html );
	foreach ( $tags as $tag ) {
	$elements = $dom->getElementsByTagName( $tag );
	foreach ( $elements as $element ) {

	// Bail if we already have an explicitly defined class value.
	if ( $exception && false !== strpos( $element->getAttribute( 'class' ), $exception ) ) {
	continue;
	}

	$element->setAttribute( 'class', trim( $element->getAttribute( 'class' ) . ' ' . $class ) );
	}
	}
	$content = preg_replace( '/<\/?(!doctype\|html\|head\|meta\|body)[^>]*>/im', '', $dom->saveHTML() );
	libxml_use_internal_errors( false );

	return $content;
	}