<?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();

?>

00:05