PHP function to calculate the time between two timestamps (hours)_PHP tutorial

/* Author : Yang Yu yangyu@sina.cn */
//Enter two timestamps and calculate the difference, which is the number of hours. If 2:10 is returned, it means that the difference between the two entered times is 2 hours and 10 minutes.
function hours_min($start_time,$end_time){
if (strtotime($start_time) > strtotime($end_time)) list($start_time, $end_time) = array($end_time, $start_time);
$sec = $start_time - $end_time;
$sec = round($sec/60);
$min = str_pad($sec%60, 2, 0, STR_PAD_LEFT);
$hours_min = floor($sec/60);
$min != 0 && $hours_min .= ':'.$min;
return $hours_min;
}

function get_date_different( ){
var _date_1 = document.getElementById('date1').value.replace(/(^s*)|(s*$)/g,'');
var _date_2 = document.getElementById( 'date2').value.replace(/(^s*)|(s*$)/g,'');
_date_1 = new Date(_date_1);
_date_2 = new Date(_date_2);
var days= _date_2.getTime() - _date_1.getTime();
var time = parseInt(days / (1000 * 60 * 60 * 24));
document.getElementById('content') .innerHTML = 'The difference between the two dates is'+time+' days! ';}