随机生成16位字符串

function generate_str($length = 16 ) {
// 密码字符集，可任意添加你需要的字符
    $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $str = "";
    for ( $i = 0; $i < $length; $i++ )
    {
        $str .= $chars[ mt_rand(0, strlen($chars) - 1) ];
    }
    return $str;
}