Implementation of merge-sort in PHP.

mergesort.php

<?php

// declare example array to be sorted
$arr1 = array(4,16,32,33,49,3,2,7,4,88);

// sort array and print
print_r(mergeRec($arr1));


// Recrusive mergesort function
function mergeRec($list) {

  // base case
	if(sizeof($list) == 1) { return $list; }

	// find middle index
	$middle = sizeof($list) / 2;


	$leftList = mergeRec(array_slice($list, 0, $middle));
	$rightList = mergeRec(array_slice($list, $middle));

	return merge($leftList, $rightList);

}

// merge 2 sorted arrays
function merge($leftList, $rightList) {

	$mergedArr = array();

	// while both arrays are still full
	while(sizeof($rightList) > 0 && sizeof($leftList))
	{
		if($leftList[0] > $rightList[0]) {
			array_push($mergedArr, array_shift($rightList));
		}
		else {
			array_push($mergedArr, array_shift($leftList));
		}
	}
	while(sizeof($rightList) > 0) {
		array_push($mergedArr, array_shift($rightList));
	}
	while(sizeof($leftList) > 0) {
		array_push($mergedArr, array_shift($leftList));
	}
	return $mergedArr;
}




?>

Please beware that array_shift is O(N), since it recalculates the keys.
So that will slow down your merge sort.

pls tell us more about O(N) why is so important here