public function compareMTimes($file1, $file2)
    {
        $mtime1 = filemtime($file1);
        $mtime2 = filemtime($file2);

        if ($mtime1 === false) { // FAILED. Log and return err.
            // Add error from php to end of log message. $php_errormsg.
            $msg = "FileSystem::compareMTimes() FAILED. Cannot can not get modified time of $file1.";
            throw new Exception($msg);
        } elseif ($mtime2 === false) { // FAILED. Log and return err.
            // Add error from php to end of log message. $php_errormsg.
            $msg = "FileSystem::compareMTimes() FAILED. Cannot can not get modified time of $file2.";
            throw new Exception($msg);
        } else { // Worked. Log and return compare.
            // Compare mtimes.
            if ($mtime1 == $mtime2) {
                return 0;
            } else {
                return ($mtime1 < $mtime2) ? -1 : 1;
            } // end compare
        }
    }