PHP产生随机字符串

/**
 * 产生随机字符串函数
 *
 * @param int $length 随机字符串位数
 * @return string
 */
function random($length)
{
    $hash = '';
    $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz';
    $max = strlen($chars) - 1;
    mt_srand((double)microtime() * 1000000);
    for ($i = 0; $i < $length; $i++)
    {
        $hash .= $chars[mt_rand(0, $max)];
    }
    return $hash;
}