“Static constructors” are a concept a lot of object-oriented programming languages support - although, unfortunately, PHP does not.
If you’re not familiar with the concept, a static constructor is just a method the developer can define on a class which can be used to initialise any static properties, or to perform any actions that only need to be performed only once for the given class. The method is only called once as the class is needed.
The C# guide about static constructors is a good resource to see how another language handles these, however we’ll cover the important bits below.