So I have a client who uses PPC and Organic SEO to drive traffic.  Once a user gets to the site, the most common action is to fill out a form.

I have Auto Tagging enabled in Adwords so all PPC clicks have "gclid" in the url.

I am also grabbing the Google Cookie and parsing it out to determine if the visitor is PPC or Organic.

So I have an if statement in my form code to pass PPC vs organic through the form for tracking purposes:

$url = 'http://' . $_SERVER['SERVER_NAME'] . $_SERVER['REQUEST_URI'];

if (false !== strpos($url,'gclid'))  { ?>

Essentially this is saying:

If gclid is in the url, or if the cookie contains PPC 
set the Web field as PPC Else
Set it as Organic.

this is working about 80% of the time. I am trying to raise the accuracy.  any ideas where I might be missing some data?

thanks