How to make sure object method returns expected object?

Put the whole thing under test. If one of your tests specifies that the return type must be x, hopefully the test would fail when using the new implementation. Beyond the test failing, hopefully the developers would use the tests as developer documentation and see what the intended behavior is.

No, on the language level it's not possible to constrain a function's return value. Only the function arguments' type hints are allowed — php.net/manual/en/language.oop5.typehint... What you could do, is making sure that the wrong type of the returned value is caught early. Say, in the IOC container startup code.

You instantiate your DbInterface, and check the return type of run(). If it's wrong, throw an exception.

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