What are intersection types in PHP 8.1?

Intersection types (PHP 8.1) allow declaring that a value must satisfy multiple type constraints simultaneously, using the & operator. Example: function process(Iterator&Countable $collection): void — the argument must implement both Iterator AND Countable. This is the opposite of union types (|, introduced in PHP 8.0), which accept any one of the specified types. Intersection types can only be used with class and interface names — not with primitive types like int, string, or null. They improve type safety for code that requires objects with multiple capabilities, eliminating the need to create a combined interface just to express the type constraint. Pure intersection types (A&B) cannot be mixed with union types yet (that is a future proposal).