PHP预备语句错误

所以当用户登录时，我试图更新数据库中的一些东西，似乎一切都很好，除了最后一部分，我得到的输出是:

" fatal error unknown" and " command out of sync;你现在不能运行这个命令"

下面是我的代码:

<?php
$user = $_POST['username'];
$pass = $_POST['password'];
$connect = mysqli_connect("localhost","myuser","mypass","somedb");
$query = "SELECT `password` FROM `userauth` WHERE `username`=?";
if($stmt = $connect->prepare($query)){
    $stmt->bind_param('s',$user);
    $stmt->execute();
    $stmt->bind_result($tmp_pass);
    $stmt->fetch();
    if($tmp_pass===$pass){
        $letters = array('a','b','*','x','e','d','z','p','@','#');
        $letter_key="";
        for($i=0;$i<sizeof($letters);$i++){
            $letter_key=$letter_key.$letters[rand(1,sizeof($letters-1))];
        }
        $key = rand(1341163,9999999);
        $key2 = rand(3541,9999);
        $complete_key = $key.$letter_key.$key2;
        setcookie("key",$complete_key);
        setcookie("user",$user);
        $query_auth = "UPDATE `userauth` SET `auth_key`=? WHERE `username`=?";
        if($stmt_2 = $connect->prepare($query_auth)){
            $stmt_2->bind_param('ss',$complete_key,$user);
            $stmt_2->execute();
        }else{
            echo "Critical error, unknown ".mysqli_error($connect);
            exit;
        }
        echo "success";
    }else{
        echo "Error Invalid Username or Password";
    }
}else{
    "Database link error";
}
?>

谁能解释一下可能是什么问题?是的，我所有的字段都是字符串