Runtime checks are what really differentiates PHP's approach from other languages. It gives the type signatures teeth (ensuring they don't lie) and keeps dynamic typing under control (types enforced at function boundaries), which is really important in a language where many operators and functions will return one type most of the time and a different type the rest of the time. It also means type information can be relied upon by the compiler to produce more optimal code. In this respect it has a particular edge over TypeScript, which throws away all the types when producing JavaScript and thus the JS engine has to try to reconstruct them if it wants to do optimisations.