To create a Singleton class, simply add "extends Singleton" to your class definition and add $this->checkSingletonConstraint() as the first line in your constructor.
Note: this class is abstract, because it extends an abstract super-class without implementing all abstract functions - NOT because it declares any abstract members itself.
Terminates the program and issues an E_USER_ERROR if singleton constraint is breached. (if the constructor is called and another instance of this class already exists)