How do I check if the URL accessing the page is the original URL or if it's a CNAME record.
For example, I have a site domain.com. Users can setup a CNAME where they link www.otherdomain.com to our site domain.com.
From our server point of view, how can I detect if the URL accessing the page is the actual one (domain.com) or the CNAME one (www.otherdomain.com)?
I think you have to query a DNS to know it, since this information does not usually come in request headers.
Take a look at the dns_get_record and checkdnsrr functions in PHP documentation.
You can use PHPDNS class library to find out what kind of DNS record is a given url.
$answer->results[x]->typeid //Textual record type ID (A, MX, CNAME etc)
If you're checking on URLs, you can use $_SERVER['HTTP_HOST'] in PHP. That's the requested host from the URL (eg. http://otherdomain.com/blah.php would make the server var be otherdomain.com). $_SERVER['SERVER_NAME'] is the configured servername in the httpd.conf.
If HTTP_HOST != SERVER_NAME, the remote user is almost certainly using an alias of some sort to access the site.
