更新查询到mysqli预处理语句问题

Question

尝试使用Prepared Statements更新我的Mysqli查询（出于安全原因，因为我需要将变量用于查询），没有运气可以让它工作，任何想法？ 此外，还可以将表名$ this->userTbl替换为?，并在$smtp上声明吗？

function __construct(){
        if(!isset($this->db)){
            // Connect to the database
            $conn = new mysqli($this->dbhost, $this->dbUsername, $this->dbPassword, $this->dbName);
            if($conn->connect_error){
                die("Failed to connect with MySQL: " . $conn->connect_error);
            }else{
                $this->db = $conn;
            }
        }
    }
    function checkUser($userData = array()){
        if(!empty($userData)){
            // Check whether user data already exists in database
            //$prevQuery = "SELECT * FROM ".$this->userTbl." WHERE oauth_provider = '".$userData['oauth_provider']."' AND oauth_uid = '".$userData['oauth_uid']."'"; //<<-- THIS WAS WORKING
            $stmt = $prevQuery->prepare = "SELECT * FROM ".$this->userTbl." WHERE oauth_provider=? AND oauth_uid=?";
            $stmt->bind_param("ss", $userData['oauth_provider'], $userData['oauth_uid']);
            $stmt->execute();
            $prevResult = $this->db->query($prevQuery);
            if($prevResult->num_rows > 0){
                // USER EXIST!! , update data
$query = "UPDATE ".$this->userTbl." SET ...
}else{
//NEW USER
$query = "INSERT INTO ".$this->userTbl." SET ...
}