Minor Description
As you're currently switching to PHP5 OOP code, I would suggest to replace this ugly global constant by a static attribute of the LogManager class i.e. LogManager::$configFile = ... Or maybe even a constructor/factory argument
This would have the additional benefit that it could be set by some kind of Dependency Injection framework like Spring for Java, although I don't know of such is even existing for PHP 
But it might come in the future.