There exists a PHP HowTo which explains how to program your own Whois Search. This Howto describes the trivial Whois Search and an extended Whois Search, which is similar to the script used on this page.
There are many usefull classes which can be used freely. For example the HTTP_Request class can be used for any HTTP 1.1 GET Request with compressed data transfer.
Unfortunatly this HowTo exists only in german and needs to be translated into English (by the way, the complete site needs to be corrected into good English). If you want to translate a part feel free to contact me.