PHP’s Type System Dissected

As any programming language, PHP has a type system.
Its type system has evolved over the years to become more complete and robust. PHP also has a notion of a subtype following Liskov’s Substitution Principle, or LSP.

But what is a type system, and why do we use Liskov’s Substitution Principle as PHP’s notion of a subtype?

In this talk we will answer those questions by using some formal theory and explain PHP’s type system, and explore what potential future may exist for PHP’s type system.