Well all the files are PHP files but the script language="JavaScript"
Sorry, but that still doesn't make allot of sense. Php isn't like asp where it can be written in several languages. Php is written in php, thats it.
More than likely the chat you have found uses both php and javascript, ,aybe even in an Ajax type combination. Im not sure how exactly .NET works, but php needs to be run inside of files with the .php extension. You can however change your server to interpret any extension as php, but changing this to let .aspx files be run as php would surely break the rest of your application.
Basicaly, I think your going to need to make this a stand alone php page, or, do as other have suggested and try and find a chat written in .NET.