Modify Post Data (Title) on Submission

I'm working on a form that creates a Post where I want the post title to be dynamically generated from a number of other form fields (essentially, the title is a tracking code made up of other codes generated by other fields).

Using a content template on the Post Title does exactly what I want except for one thing: depending on the values a certain field (Form Field 7), the way the title is constructed changes. So I need to write a custom function that will create the Post Title based on some if statements.

Due to the way my site is managed, I prefer to do this as a plugin rather than in the themes functions.php file.

Here's what I've written:

add_filter("gform_post_data", "create_proposal_title", 10, 2);

function create_proposal_title($post_data, $entry) {

if ($entry['7'] == 'NEWPRG') {
    $post_data['post_title'] = $entry['7']."-".$entry['17']."-".$entry['3']."-".$entry['9']."-".$entry['id'];

    }
    return $post_data;

}

However, this isn't working. I'm not sure what I'm doing incorrectly. Is there anything obvious with my syntax of use of the filter?

Ah! Okay. I can try putting it in functions. The site is a big, institutional multisite installation so functions is already a beast. That's why I was trying to avoid it. But I see what you mean.

Okay, I moved the code into functions.php, and it's still not working. The post gets created, but the title is not set correctly.

One question: for the if statement where I'm testing the value of form field #7 ($entry['7']), should I be testing against the Label for the selection of the Value? I assumed Value and that's what the code above reflects. (I've actually tried both, and it doesn't seem to help.)

Oops, I meant to mention: My ultimate goal is for the post title to be a concatenation of several other form field values, but, for testing purposes, I'm just using a string right now.

The title I'm getting when I use this function is just "Untitled."