INSERT into relational db problem

I'm settin up a basic catalogue/ ecommerce site with a db structure along these lines:

table 'films'
---------
title_id
title
etc...

table 'genres'
---------
genre_id
genre

table 'film_genre_links'
---------
title_id
genre_id

I'm using PHP to display the relational info for each title on a page - this works okay. How do I do it in reverse? How do I insert title & genre id's into the link table using the genre names?

Along the lines of "INSERT INTO film_genre_links (title_id,genre_id) VALUES ($title_id,$genre_id) WHERE genre.genre=$genre"

Sorry if this is vague or on the wrong forum - newbie.
The $variables relate to text boxes or check boxes on an html form

I thank you

Your form variables are sent back to the server either by GET (as part of the URL) or by POST (as part of the request body). You can access these variables and do the INSERT query with them.

Since you're using PHP, you access your form elements thus:

$genre = $_GET['genre'];

or

$genre = $_POST['genre'];

Depending on how you've set up your form.