Inserting a row in a mysql table. It doesn't work?

Question

I'm kind of new to PHP and I'm trying to build a REST API for an app. For some reason I got stuck at something that should be trivial. What I want to do is to be able to insert a new row into the Score-table which is defined like this:

CREATE TABLE Scores(
        score_id INT AUTO_INCREMENT,
        user_id INT NOT NULL,
        item_id INT,
        score DOUBLE UNSIGNED,
        PRIMARY KEY (score_id),
        FOREIGN KEY (item_id) REFERENCES Items(internal_id),
        FOREIGN KEY (user_id) REFERENCES Users(user_id) ON DELETE CASCADE
        );

Below is the PHP code I use. The function setScore is a member in a class which also has the db and response -members which you will see. But I have debugged the code enough to see that everything apart form the below function works.

The POST-variables have the correct values and the sendResponse(200, "SCORE_INSERTED_SUCCESSFULLY") is executed properly. And I do not try to insert duplicate rows.

The problem is that I don't get a new row in the Score-table. However, strangely enough I can see that the primary key is auto incremented each time I try to insert a new row even though no new row appears!

I can insert the row manually by running the exact same query in mysql manually. For example, this works if I run it manually in mysql:

INSERT INTO Scores (user_id, item_id, score) VALUES (1,2,6.0);

So why does the below not work?

function setScore() {       
        if (isset($_POST["user_id"]) && isset($_POST["item_id"]) && isset($_POST["score"]))
            {
                $user = $_POST["user_id"];
                $item = $_POST["item_id"];                
                $score = $_POST["score"];

                $query = 'INSERT INTO Scores (user_id, item_id, score) VALUES (?,?,?)';
                $stmt = $this->db->prepare($query) or trigger_error($this->db->error."[$query]");
                $stmt->bind_param("iid", $user, $item, $score);
                $stmt->execute();
                $stmt->close();                
                $this->response->sendResponse(200, "SCORE_INSERTED_SUCCESSFULLY");
                return true;
            }
        $this->response->sendResponse(400, "Invalid request");
        return false;
    }

I really appreciate any feedback, this is driving me mad!

Thanks in advance

EDIT:

This is the value of the $stmt-variable before execute:

mysqli_stmt Object
(
    [affected_rows] => 0
    [insert_id] => 0
    [num_rows] => 0
    [param_count] => 3
    [field_count] => 0
    [errno] => 0
    [error] => 
    [error_list] => Array
        (
        )

    [sqlstate] => 00000
    [id] => 1
)

and after execute:

mysqli_stmt Object
(
    [affected_rows] => 1
    [insert_id] => 30
    [num_rows] => 0
    [param_count] => 3
    [field_count] => 0
    [errno] => 0
    [error] => 
    [error_list] => Array
        (
        )
    [sqlstate] => 00000
    [id] => 1
)

Answer 1

But I have debugged the code enough to see that everything apart form the below function works.

I had to eat my own words this time. It turned out I should have posted the whole class because I had forgot that I set $this->db->autocommit(FALSE); . When I commited everything worked as expected!

Thank you for all your comments