You should use the DateTime class but if you use PHP version older than 5.2 you can do it like that:
function convert_time_by_seconds($interval) {
if($interval < 0) {
return false;
}
$seconds_n_miliseconds = explode('.', $interval);
$interval = $seconds_n_miliseconds[0];
$hours = floor($interval / (60*60));
$minutes = floor(($interval - ($hours*60*60)) / 60);
$seconds = floor(($interval - ($hours*60*60)) - ($minutes*60));
$ms = empty($seconds_n_miliseconds[1]) ? 0 : $seconds_n_miliseconds[1];
return array('h' => $hours, 'm' => $minutes, 's' => $seconds, 'ms' => $ms);
}
$time_diff = time() - strtotime('2014-07-17 14:23:51');
$result = convert_time_by_seconds($time_diff);
var_dump($result);