Split time in to time slots - PHP

function prepare_time_slots($starttime, $endtime, $duration){
	 
	$time_slots = array();
	$start_time    = strtotime($starttime); //change to strtotime
	$end_time      = strtotime($endtime); //change to strtotime
	 
	$add_mins  = $duration * 60;
	 
	while ($start_time <= $end_time) // loop between time
	{
	   $time_slots[] = date("H:i", $start_time);
	   $start_time += $add_mins; // to check endtime
	}

	return $time_slots;
}

$starttime = '9:00';  // your start time
$endtime = '21:00';  // End time
$duration = '30';  // split by 30 mins

$time_slots = prepare_time_slots($starttime, $endtime, $duration);

echo '<pre>';
print_r($time_slots);
echo '</pre>';

Array
(
	[0] => 09:00
	[1] => 09:30
	[2] => 10:00
	[3] => 10:30
	[4] => 11:00
	[5] => 11:30
	[6] => 12:00
	[7] => 12:30
	[8] => 13:00
	[9] => 13:30
	[10] => 14:00
	[11] => 14:30
	[12] => 15:00
	[13] => 15:30
	[14] => 16:00
	[15] => 16:30
	[16] => 17:00
	[17] => 17:30
	[18] => 18:00
	[19] => 18:30
	[20] => 19:00
	[21] => 19:30
	[22] => 20:00
	[23] => 20:30
	[24] => 21:00
)