public static function undefined_function( $function_name ) {

		if ( \function_exists( $function_name ) ) {
			return new \Twig_Function(
				$function_name,
				function () use ( $function_name ) {

					ob_start();
					$return = \call_user_func_array( $function_name, \func_get_args() );
					$echo   = ob_get_clean();

					return empty( $echo ) ? $return : $echo;
				},
				array( 'is_safe' => array( 'all' ) )
			);
		}

		return false;
	}