title: Passing data to "thank you" page when form action goes to third party site
body: <p>I have a web form that collects a user's zip code as one of the field and posts to a third party site...</p>
  
  <pre><code>&lt;form name = "foo" action="https://thirdpartysite.com" method="post"&gt;
  </code></pre>
  
  <p>The form has a "redirect_link" field that tells the third party site to redirect to my thank you page, so the form processing happens on their side. If I were doing the form processing, I could store the zip code in a session variable before saving it to my database, and pass it to the thank you page in that way. But because the form processing is being done by someone else, I'm not sure how I can access that zip code field, store it somewhere and pass it to my thank you page. I'm thinking maybe I could do it by using javascript to grab the zip code and storing it in a php session variable? I'm not sure if that would work, or how to do it.</p>