PHP has a type system, and it has quietly become one of the more interesting ones among dynamic languages. It grew by accretion over two decades: scalar types, nullable types, union and intersection types, never, true, and a notion of subtyping built on Liskov's Substitution Principle.

This talk starts from the formal question of what a type system actually is, then uses that vocabulary to explain PHP's. Why LSP is the rule that governs subtyping, what variance means for the signatures you write every day, and where PHP's system is genuinely sound as opposed to pragmatically compromised.

We finish by looking forward: what PHP's type system could still gain, what each addition would cost, and what is realistically on the table. And there's a practical reason to care right now. Types are the clearest signal a codebase gives a model about what it may do, so knowing what they actually guarantee is worth more than it was two years ago.