Mergesort in PHP

gistfile1.php

<?php
function merge_sort(Array $ar)
{
    if (count($ar) <= 1) return $ar;
    list($a, $b) = half($ar);
    $aa = merge_sort($a);
    $bb = merge_sort($b);
    return merge($aa, $bb);
}
function half(Array $ar)
{
    $mid = floor(count($ar) / 2);
    return array(array_slice($ar, 0, $mid), array_slice($ar, $mid));
}
function merge(Array $a, Array $b)
{
    $result = array();
    while (!(empty($a) && empty($b))) {
        switch (true) {
        case empty($a):
            $result[] = array_shift($b);
            break;
        case empty($b):
            $result[] = array_shift($a);
            break;
        case $a[0] <= $b[0]:
            $result[] = array_shift($a);
            break;
        default:
            $result[] = array_shift($b);
            break;
        }
    }
    return $result;
}

$ar = array(4,5,6,2,12,5,9);
var_dump(merge_sort($ar));
/*
array(7) {
  [0]=>
  int(2)
  [1]=>
  int(4)
  [2]=>
  int(5)
  [3]=>
  int(5)
  [4]=>
  int(6)
  [5]=>
  int(9)
  [6]=>
  int(12)
}
*/