Parsing URL's with Apache and PHP

I'm trying to setup a system where I can parse the entire URL that is given in the address bar of the broswer. My intentions are to make use of the parse_url() function supplied in PHP, but I cannot find a way to get the full address requested. Example: http://bcelenza.somedomain.com/index.php?page=blah Are there variables or methods with apache I can use to get this entire address? All I'm seeing is the HTTP Referrer and some other _SERVER vars that do not give me the whole address. The reason I am trying to do this is to parse the address bcelenza.somedomain.com to return the subdomain, bcelenza. Any ideas are certainly appreciated. Thank you, Brian