how to subtract two time in php code example

Example: substract 2 dates php

$datetime1 = strtotime('May 3, 2012 10:38:22 GMT');
$datetime2 = strtotime('06 Apr 2012 07:22:21 GMT');

$secs = $datetime2 - $datetime1;// == <seconds between the two times>
$days = $secs / 86400;