Hi Baber.
The Extreme Programming concept has been around for a few years now. Mostly it is a set of methodologies and practices used by a team of programmers to help production, cut down on bugs and to keep the customer involved at all stages of software development.
I can see it used in large software houses such as game developers (I somehow doubt microsoft parctice it) but it is not limited to any one particular language.
I can't see it being used by anyone developing applications in PHP as it is best used in large development teams. Php applications tend to be developed by small (2-3 programmer) companies or by a geographically dispersed team via cvs.
See
extremeprogramming.org for more information on the concept.
Regards, Eamonn.