php 生成随机字符串

<?php    function generateRandomString($length = 15) {        return substr(str_shuffle(str_repeat($x='0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', ceil($length/strlen($x)))), 1, $length);    }    $db   = new PDO('mysql:host=127.0.0.1;dbname=random', 'root', 'xxxxxxx');    $stmt = $db->prepare("INSERT INTO random(`random`) VALUES(?);");    for ($i = 0; $i <= 150000; $i++) {        $stmt->execute(array(generateRandomString()));    }