php - Insert data to table then insert the newly added id to the foreign key

I'm studying php myself and I want my table to get the id of 2 table, the first one is from the new title I added, the second one is from the genre table, so I can put up a lot of category for the title but I can't find any tutorial and it's not in w3school. Don't mind the lack of security as I haven't tackled it yet

My tables(db name is testdb:

    title
    ---------------------
    |title_id|title_name|

    ---------------------

    genre
    ---------------------
    |genre_id|genre_name|
    ---------------------
    |1       | action   |
    |2       | horror   |
    ---------------------


    filter
    -----------------------------
    |filter_id|genre_id|title_id|
    -----------------------------

My php connect is (saved as con.php):

DEFINE ('DB_HOST', 'localhost');

DEFINE ('DB_USER', 'root');
DEFINE ('DB_PSWD', '****');
DEFINE ('DB_NAME', 'testdb');

$dbtest = mysqli_connect(DB_HOST, DB_USER, DB_PSWD, DB_NAME);
?>

PHP code is:

if (isset($_POST['submit'])) 
{
include_once('con.php');
$ntitle = $_POST['newtitle'];
$sqlinsert = "INSERT INTO title (title_name) VALUES ('$ntitle')";
if (!mysqli_query($dbtest, $sqlinsert)) 
{
mysqli_error($dbtest);
} else {

echo ('1recordadded');
}
}


?>

My form:

legend>New Content:
Title: 



action

horror