/**
 * mtime()  Get the modified time of a directory or file. For directories,
 *          it gets the modified time of the most recently modified file.
 *
 * @param   string       $path     Full path to directory or file.
 * @param   string       $format   Date string for use with date()
 * @return  number|string|null
 */
function mtime ( $path, $format = null  ) {
    $time = null;
    if ( \is_string($path) && \is_readable($path) ) {
        if ( \is_dir($path) ) {
            $path = \rtrim($path, '/');
            foreach ( \scandir($path) as $file ) {
                if ( '.' !== \substr($file, 0, 1) ) {
                    $temp = mtime($path . '/' . $file);
                    $temp > $time and $time = $temp;
                }
            }
        } elseif ( \file_exists($path) ) {
            $time = \filemtime($path);
        }
    }
    return $time && $format ? \date($format, $time) : $time;
}

Get modified time of a directory or file. For directories: it gets the modified time of the most recently modified file.

mtime: modified time

PHP