Returns a reference to a concrete Log instance of log_type, only creating a new instance if no Log instance with the same parameters currently exists.
You should use this if there are multiple places you might create a logger, you don't want to create multiple loggers, and you don't want to check for the existance of one each time. The singleton pattern does all the checking work for you.
Note: You MUST call this method with the $var = &Log::singleton()syntax. Without the ampersand (&) in front of the method name, you will not get a reference; you will get a copy.