Question: Does PHP have global variables that can be modified by one running script and read by another?

Answer: 
Each request is handled by a php instance of its own. Global variables in php are only accessible from within the same php instance. However you can use something like the [memchached module](http://docs.php.net/memcached) to share data between different instances (which should usually be faster than writing the data to the filesystem).
Not as such, but you can use cookies or sessions to maintain data for duration of a user's browsing experience, or you can write to a database or file on-disk if the information needs to persist beyond that.