in_place_quicksort.php

<?php

function swap(array &$array, $k1, $k2) {
    $temp = $array[$k1];
    $array[$k1] = $array[$k2];
    $array[$k2] = $temp;
}

function partition(array &$array, $left, $right, $pivot) {
    $pValue = $array[$pivot];
    swap($array, $right, $pivot);
    $storeIdx = $left;
    for ($i = $left; $i < $right; $i++) {
        if ($array[$i] < $pValue) {
            swap($array, $i, $storeIdx);
            $storeIdx++;
        }
    }
    swap($array, $storeIdx, $right);
    return $storeIdx;
}

function quicksort2(array &$array, $left = null, $right = null) {
    if (is_null($left) || is_null($right)) {
        $left = 0;
        $right = count($array) - 1;
    }
    if ($left < $right) {
        $pivot = $left + floor(($right - $left) / 2);
        $pivot = partition($array, $left, $right, $pivot);
        quicksort2($array, $left, $pivot - 1);
        quicksort2($array, $pivot + 1, $right);
    }
}

recursive_quicksort.php

<?php

function quicksort(array $array) {
    $count = count($array);
    if ($count <= 1) {
        return $array;
    }
    $left = array();
    $right = array();
    $key = floor($count / 2);
    $value = $array[$key];
    for ($i = 0; $i < $count; $i++) {
        if ($key != $i) {
            if ($array[$i] < $value) {
                $left[] = $array[$i];
            } else {
                $right[] = $array[$i];
            }
        }
    }
    return array_merge(quicksort($left), array($value), quicksort($right));
}