It would be useful if, when placing an order or submitting a form, we could reliably include the visitor's referrer with the request. This is not always simple because, for example, the user could have found us via Adwords, left the site and come back by typing in the address. In this case, we would want to know the Adwords campaign for this specific visitor. Google Events don't fit well with our workflow.
Other questions have tried to use Javascript and apparently that is not possible. I noticed that Google sets a _ga cookie which we can read - it gives us an ID that looks like this: GA1.2.123456789.1234567890
. Can we use PHP to extract the referrer according to Google from that ID?