How To Get Client's Information?

I'm working on a website hit tracking appilication, in php we can use REMOTE_ADDR to retrieve the client's IP address, but how do I get its isp server (domain)?

For example, the domain: pcp08721939pcs.norstn01.pa.comcast.net is associated with IP: 68.32.198.43

How can I use php functions to get 'pcp08721939pcs.norstn01.pa.comcast.net'?

Anyone can help me out?