Get the directory's modified time based on when the last file or directory within it was updated

dirmtime.php

<?php

/**
 * Get the directory's modified time based on when the last file within it was updated
 *
 * @param  string   Which directory to look within
 * @param  string   A regular expression to match files
 *
 * @return int      The timestamp
 */
function dirmtime($path = '', $regex = null)
{
    $directory = new RecursiveDirectoryIterator($path);
    $iterator  = new RecursiveIteratorIterator($directory);

    if ($regex) {
        $iterator = new RegexIterator($iterator, $regex, RecursiveRegexIterator::GET_MATCH);
    }

    $mtimes = [];

    foreach ($iterator as $file) {
        if (is_array($file)) {
            foreach ($file as $f) {
                $mtimes[] = filemtime($f);
            }

            continue;
        }

        $mtimes[] = filemtime($file);
    }

    return empty($mtimes) ? null : max($mtimes);
}