該方法本人已測試通過，如各位網友有更好的方法，歡迎分享。

function GetRandStr($len) {
    $chars = array(
        'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','0','1','2','3','4','5','6','7','8','9');
    $charsLen = count($chars) - 1;
    shuffle($chars);
    $output = '';
    for ($i = 0; $i < $len; $i++) {
        $output .= $chars[mt_rand(0, $charsLen)];
    }
    return $output;
}

echo GetRandStr(5); //5為隨機字串個數