PHP's type system dissected

PHP has added more and more types in the last couple of versions. But what is the meaning of a type, a type system, and a subtyping relation? In this talk, we will explain those words, and see what comprises the PHP type system, how it checks for types, how it uses and implements Liskov's Subtitution Principle (LSP) for it's subtyping relation, and how the strict_type declare affects PHP's type system.