PHP生成自定义长度随机字符串

function getRandChar($length)
{
    $str = null;
    $strPol = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz0123456789";
    $max = strlen($strPol) - 1;

    for ($i = 0;$i < $length;$i++) {
        $str .= $strPol[rand(0, $max)];
    }

    return $str;
}