I'm a novice in PHP and I would like to know on how exactly could I define said row. Any helps/tips are appreciated. Thank you in advanced!

function add_review() {
    if(isset($_POST['add_review'])){

       (THE UNDEFINED ROW) **$product_id        = $row['product_id'];**
        $review_name       = escape_string($_POST['review_name']);
        $review_email      = escape_string($_POST['review_email']); 
        $review_content    = escape_string($_POST['review_content']);
        $review_created    = date("Y-m-d H:i:s");
        $rating            = escape_string($_POST['rating']);

        $review_query = query("INSERT INTO reviews(review_id, product_id, review_name, review_email, review_content, review_created, rating) VALUES('NULL', '$product_id', '$review_name', '$review_email' , '$review_content' , '$review_created' , '$rating')");

        confirm_query($review_query);

        set_message("Thank you for the feedback!");
    }}

I think the reason you haven't received any responses is because your question is confusing. What do you mean by define said row? Which row? Define in which way?

I see line 4 you have (THE UNDEFINED ROW) but I'm not sure what you are trying to do?