Domain

Using PHP:

How can I know that domain is registered or not ?

You can do a gethostbyname() call. If it returns a valid IP then it's registered.

that won't work if the domain is registered but not up yet. a lot of people just buy the domains but never put a site on them.

the best bet is to foward the user to register.com or something and have them search.

http://www.php.net/manual/en/function.checkdnsrr.php