Insert php works but the data is not save in database

I want to insert data into database with ajax and I got message in console that it does work also no errors but the data is not inserting into database.

Php code:

<?php 

include 'connection.php';


if($_POST['action'] == 'rating')
{
    echo "work";

    $user_name = $_POST['user_name'];
    $rating_index = $_POST['rating_index'];
    $user_review = $_POST['user_review'];

    $stmt = $conn->prepare("INSERT INTO review_table
    (review_id, user_name, user_rating, user_review, datetime) 
    VALUES (NULL, ?, ?, ?, NOW())");
    $stmt->bind_param('sis',  $user_name,  $user_rating, $user_review);
    $stmt->execute();
}
?>

and like I said I have echo “work” printed in console so it is connecting with ajax and also I checked if variable are set correctly and they are.

ajax code:

 $.ajax({
                                url:"rating-data.php",
                                method:"POST",
                                data: {
                                    action:'rating',
                                    rating_index: rating_index,
                                    user_name: user_name,
                                    user_review : user_review
                                },
                                success:function(data)
                                {
                                    $('#review_modal').modal('hide');
                                    // load_rating_data();
                                    console.log(data);
                                }
                            })