PHP随机生成四位字符串

public function RandStr()
{
    $arr1 = array_merge(range('A', 'Z'), range(0, 9), range('a', 'z')); // 生成数组且合并为arr1
    shuffle($arr1); // 打乱顺序
    $arr2 = array_rand($arr1, 4); // arr1随机取四位生成新数组
    $str = "";
    foreach ($arr2 as $index) {
        $str .= $arr1[$index];
    }
    return $str;
}