Hi Dado,
HTTP Client? Are you referring to the browser user agent? If so, you can use get_browser(), which returns an object with a number of properties containing information about that browser, or you can use the phpsniff(), which can be downloaded from the following site:
http://sourceforge.net/projects/phpsniff
Additionally, you can do your own by querying
the predefined variable $HTTP_USER_AGENT.
Regards,
David