Add a Class to a WordPress Metabox

add-class-to-wp-metabox

/**
 * The simplistic way to add a custom class to a specific metabox
 *
 * @param   array   $classes    The current classes on the metabox
 * @return  array               The modified classes on the metabox
 */
function add_metabox_classes( $classes = array() ) {

    return array_push( $classes,  sanitize_html_class( 'my-custom-class' ) );

} // add_metabox_classes()

/**
 * {post_type_name}     The name of the post type
 * {metabox_id}         The ID attribute of the metabox
 */
add_filter( 'postbox_classes_{post_type_name}_{metabox_id}, 'add_metabox_classes' );

/**
 * The more thorough way to add a custom class to a specific metabox
 *
 * Uses the same add_filter call, but its easier to add additional classes
 * and it checks if new class is already there.
 *
 * @param   array   $classes    The current classes on the metabox
 * @return  array               The modified classes on the metabox
 */
function add_class( $classes = array() ) {

	$add_classes = array( 'new_class1', 'new_class2' );

	foreach ( $add_classes as $class ) {

		if ( ! in_array( $class, $classes ) ) {

			$classes[] = sanitize_html_class( $class );

		}

	} // End of foreach loop

	return $classes;

} // add_class()