random_string.php

<?php
if (!function_exists('randcomString')) {
    /**
     * @param number
     * @return string
     */
    function randcomString($length = 16)
    {
        $capital = substr(str_shuffle('ABCDEFGHIJKLMNOPQRSTUVWXYZ'), 0, 1);
        $small   = substr(str_shuffle('abcdefghijklmnopqrstuvwxyz'), 0, 1);
        $number  = substr(str_shuffle('0123456789'), 0, 1);
        $special = substr(str_shuffle("!#$%&*+-<=>@_|"), 0, 1);
        $pool    = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!#$%&*+-<=>@_|';
        return substr(str_shuffle(str_repeat($pool, 5)), 0, $length - 4) . $capital . $small . $number . $special;
    }
}