$app of type object<Pimple\Container> is not a sub-type of object<Silex\Application>. It seems like you assume a child class of the class Pimple\Container to be always present.
This check looks for parameters that are defined as one type in their type
hint or doc comment but seem to be used as a narrower type, i.e an
implementation of an interface or a subclass.
Consider changing the type of the parameter or doing an instanceof check before assuming
your parameter is of the expected type.
This check looks from parameters that have been defined for a function or method, but which are not used in the method body.