CPA Postback

Might sound noobish but I am trying to make a GPT script and I am having a little trouble setting up the postback script

I am signed up at adscend media, redfirenetwork and adwork media

My problem is getting a custom variable (User ID) to be forwarded to the postback script, I want to identify who completed the survey by the user ID's (that is in my database) and not their IP since they might change it. How do I do that? I asked support at redfirenetwork and adsncend media and they are totally useless when it comes to php so I am asking here if anyone knows.

I was hoping it would be as easy as just adding ?uid=<?php echo $UserID; ?> to the survey URL but it seems thats not the case
How do other gpt scripts do this?
I was thinking maybe comparing the IP to last logged in IPs (since I am tracking that) and get the user ID from there but I am not sure how well that would be..

I am sure there someone here that can help me!

Not sure about the other networks as I'm not a member, but for Adscendmedia, it looks like you'd be able to pass the $UserId variable as the sid, and then reference that through your postback script.

HTH
ND

Isn't sid at adscend media used to pass on the postback prefix? So it should always be the same otherwise it wont know what postback url to use

You would append the ?sid=$UserId to the survey URL, your base postback url will always be the same, but would include the ?sid=$UserId when completion is processed.

Survey Link would look like this - Postback link would look like this -
ND