<?php
$start_time_o = "18:00";
$end_time_o = date("H:i");
function get_time_difference(){
global $start_time_o, $end_time_o;
$start_time = explode(":", $start_time_o);
$end_time = explode(":", $end_time_o);
$start_time_stamp = mktime($start_time[0], $start_time[1], 0, 0, 0, 0);
$end_time_stamp = mktime($end_time[0], $end_time[1], 0, 0, 0, 0);
$time_difference = $end_time_stamp - $start_time_stamp;
return gmdate("H:i", $time_difference);
}
echo get_time_difference();
?>