php快速排序

<?php
function quick($arr){
if(!is_array($arr)) return false;
$length = count($arr);
if($length <= 1) return $arr;
$left = $right = array();
for($i=1;$i<$length;$i++)
{
if($arr[$i] < $arr[0])
{
$left[] = $arr[$i];
} else 
{
$right[] = $arr[$i];
}
}
$left = quick($left);
$right = quick($right);
return array_merge($left,array($arr[0]),$right);
}


$a = [21,63,1,432,143,56,11];
echo '<pre>';
var_dump(quick($a));