[+] ISO 8601 to seconds

/*
 * Convert ISO 8601 values like PT51M54S
 * to a total value of seconds.
 */
function ISO8601ToSeconds($ISO8601){
    $interval = new \DateInterval($ISO8601);
    return ($interval->d * 24 * 60 * 60) +
        ($interval->h * 60 * 60) +
        ($interval->i * 60) +
        $interval->s;
}
echo ISO8601ToSeconds('PT51M54S'); // Returns a value of 3114