Write a PHP program to sort a list of elements using Merge sort

<?php
function merge_sort($my_array){
	if(count($my_array) == 1 ) return $my_array;
	$mid = count($my_array) / 2;
    $left = array_slice($my_array, 0, $mid);
    $right = array_slice($my_array, $mid);
	$left = merge_sort($left);
	$right = merge_sort($right);
	return merge($left, $right);
}
function merge($left, $right){
	$res = array();
	while (count($left) > 0 && count($right) > 0){
		if($left[0] > $right[0]){
			$res[] = $right[0];
			$right = array_slice($right , 1);
		}else{
			$res[] = $left[0];
			$left = array_slice($left, 1);
		}
	}
	while (count($left) > 0){
		$res[] = $left[0];
		$left = array_slice($left, 1);
	}
	while (count($right) > 0){
		$res[] = $right[0];
		$right = array_slice($right, 1);
	}
	return $res;
}
$test_array = array(100, 54, 7, 2, 5, 4, 1);
echo "Original Array : ";
echo implode(', ',$test_array );
echo "\nSorted Array :";
echo implode(', ',merge_sort($test_array))."\n";
?>