Changing a field value on submit using a PHP variable

Question

Preface:

Ok, I know people are going to tell me

"use JavaScript"

To you people I'll say

"No! I don't want to!"

but I have a valid reason:

Problem overview:

I have a plugin which automatically creates a post and assigns the category "YouTube" - and gives it the post type "Video" whenever a new video is uploaded to a YouTube channel.

But in order for the post to display correctly on the website the Video ID ($_POST['_tern_wp_youtube_video']) value needs to be placed in the $_POST['pukka[_pukka_media_url]'] input field (after the standard YouTube stuff).

Where I've looked:

Everywhere that tells me it can't be done unless I use JavaScript - but like I said - I don't want to use JS because it's not really a visual thing I'm trying to fix - it's a database thing.

What I've tried:

I'm kind of half way through this. I've collected all the information I need, I just need to find the way to manipulate the other fields value (without JavaScript):

add_action( 'publish_post', 'default_post_type' );
function default_post_type( $post_id ) {
    if ( in_category( 'YouTube' ) ) {
        // Change post_format to video
        set_post_format( $post_id, 'video' );

        // Get and combine values to create the YouTube watch URL
        $youtube_pre = "http://www.youtube.com/watch?v="; // Set the YouTube watch URL prefix
        $youtube_id = $_POST['_tern_wp_youtube_video']; // Get value of YouTube Video ID field
        $youtube_value = $youtube_pre.$youtube_id; // Combine the two
    }
} 

So I've got the value that I want to enter ($youtube_value), but I can't figure out how to do it with PHP. I was thinking to use str_replace() - but that didn't work.