PHP and htaccess files are all server related, nothing to do with the arduino or xbee.
PHP is a script that runs on the server,
htaccess files tell the server certain things about the directories that contain them.
any folder in your public_html or www folders can have permissions set by simply creating a **.**htaccess file. Google is littered with examples of stuff that can go in them.
Search for php authentication tutorial
or htaccess authentication.
Read up on the basics of how to use PHP and how it works, the syntax structure is similar to c++.