This isn't PHP. This is what's known as an .htaccess file (hence the name). It is a local configuration file that affects the directory it is placed in, and those beneath it.
The posted code tells Apache that if the file extension is .php or .xml, to tell the browser not to cache the content sent. That means every time the browser sees those file names, it will send the request to the server, instead of using cached content which is already on the PC.
Without these directives, it is possible the browser will save the file content from the server until the cache control settings expire, prior to requesting a new copy.
I checked your server, and it's an Apache machine (this can be determined with Firebug).
I'm not positive this is the problem - but it is definitely possible and this is a low risk way to check.