Calculate duartion between from start times – finish time

$start_time = strtotime($start_time);
$finish_time = strtotime($finish_time);

$seconds = $finish_time – $start_time;
$hours = floor($seconds / 3600);
$mins = floor(($seconds – ($hours*3600)) / 60);
//$durations = $hours.”:”.$mins;
$durations = $hours;