It's a PHP question but I'm asking it here because the problem may have to do with the way WordPress apparently brings its modules in and out of existence. (I asked it in "Troubleshooting" but in the volume there it disappeared without a trace)
The question is, where would one declare a global so that it will survive and be seen in functions.php? I have one function there that sets a global, then another that gets the current setting of the global, but when it goes to get, there nothing there. The setting has been lost.