Cookie variable tracking

Basically, we (or our clients) run online ads on Google/LinkedIn/Facebook etc. And on the links we put in the ads, we enter "?src=xxx" at the end so we know which ad the person came from when they fill in a form on that page. The variable is submitted to our database when the form is submitted. However, if they navigate away from the first page they land on, we lose that src variable from the URL and then if they fill in a form on a different page, we don't get the tracking code, because it's not there anymore. What we need is some sort of cookie that lasts for 30 or 60 days to remember that tracking code (not necessarily in the URL) regardless of what page they visit, or whether they close the site and re-open it. I will give you an example site/page to work on with FTP and Cpanel access as required, but I need to be able to copy this on to any site. Please keep your code nicely commented and tell me first if you need to create new files so I can tell you where to put them. All of our code is stored outside of public_html for security purposes, so there should be no on-page coding.