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

Answer: 
You can actually do this using [shared memory](http://us.php.net/sem), or [APC](http://php.net/apc) (which is using shared memory itself).
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.