I have never used PHP professionally, but if you're talking about PHP 5 style interfaces, they are definitely a form of type checking, and they were introduced

I have never used PHP professionally, but if you're talking about PHP 5 style interfaces, they are definitely a form of type checking, and they were introduced with the specific purpose of facilitating object oriented programming in PHP. The point I'm getting at is that using interfaces lets you make guarantees about what methods the objects must have. I'm not really picky about what specific technical means are being used to enforce those, but passing an object of the wrong type to a method is an entirely avoidable category of errors, and I prefer them to be flagged by the compiler/interpreter/runtime/tsc/mypy/whatever as soon as possible. IME well designed interfaces are even more important than well designed classes, and dynamic languages unfortunately make it harder to enforce interfaces. It's not by chance that the first thing Typescript did was to introduce interfaces.