Make a read-only cookie

Hi,
I would like to capture a visitor's initial http_referrer when they come to our site.  I would like it to get written in a cookie, along with the sessionID.
Currently the code makes the cookie, and writes those two bits of info properly.  Also I can pull the info from the cookie.
As the user moves around the site, I don't want those cookie values to change.  That way, if they decide to fill in a contact form, we can grab their initial http_referrer from when they originally entered the site.  Is there a way to make the initial cookie read only (or a more efficient way to capture the info?)
thanks in advance for your assistance,
David 
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