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).
Global variables are bad in most programming. They're especially bad in multithreaded/multiuser systems like webapps. Avoid. If you must use global variables (rather than global constants) put them in a database with transactions guarding updates.

Since you talk about different scripts though, it sounds like what you really want is a web application framework in a more application oriented language --- something like Django (python) or Rails (ruby). These let you think of your code much more like a cohesive PROGRAM, rather than a lot of loosely connected scripts that process web requests.