Web Hosting Talk : Web Hosting Main Forums : Domain Names : recognize CNAME and original domain, how?

Dear all,
I have a problem I can't figure out how to solve. In short I want to recognize what CNAME was used to come to a website.
Two clients have a subdomain:
sub . client1 . com
sub . client2 . com
That I want to bring using CNAME to:
product . myserver . com
However, product . myserver . com will show a different product for both clients. A records are not an option because there are far to many products/clients here and the clients have their own site at www . client1 . com and www . client2 . com
So the website at product . myserver . com should somehow know the original subdomain entered (sub . client1 . com or sub . client2 . com).
How can I recognize this? Using PHP or is there a more straight forward solution here?
If it may be important, I use PLESK at myserver.com
Kind regards,
Pim Zeekoers
Netherlands

[webcertain]

you can't do it, because the DNS cname just returns an IP address to the client, it has no way to know if it came via a cname or otherwise, and even if it did, httpd headers dont supply that information,so you couldn't tell.
the best option would be to not use cnames at all, and point the subdomains to the same ip as the site, and handle things from there.