PHP function to replace array_rand(), using mt_rand()

mt_array_rand.php

<?php
function mt_array_rand(Array $array, $num = 1)
{
    $count = count($array);
    if ($num > $count) {
        return null;
    }

    $keys = array_keys($array);
    $res = [];
    for ($i = 0; $i < $num;) {
        $rand = mt_rand(0, $count - 1);
        if (isset($res[$keys[$rand]])) {
            continue;
        }
        $res[$keys[$rand]] = $i;
        $i++;
    }

    $res = array_flip($res);
    sort($res);
    return (count($res) === 1) ? reset($res) : $res;
}