How do I handle covariant parameters and not fall foul of LSP.

Hi. I have an abstract base repo and an abstract base entity. Both provide functionality appropriate to all repos/entities. I extend these to allow a specific repo to handle a specific entity. But I can't declare the specific entity type hint in the specific repo as it is in conflict. I'm hitting my head against LSP, but wanting https://en.wikipedia.org/wiki/Covariance_and_contravariance_ (computer_science)#Contravariant_method_argument_type <?php abstract class BaseEntity{} abstract class BaseRepo { public function processEntity(BaseEntity $entity){} } class Person extends BaseEntity{} class PersonRepo extends BaseRepo { public function processEntity(Person $entity) { parent::processEntity($entity); } } $person = new Person; $personRepo = new PersonRepo; $personRepo->processEntity($person); Warning: Declaration of PersonRepo::processEntity(Person $entity) should be compatible with BaseRepo::processEntity(BaseEntity $entity) I know some people 'get' this. But for me, it really seems to make next to no sense at all. I have some base behaviour for all entities. I have a base repo that can work with that base entity, regardless of how that entity is extended. If the base type were not abstract, then I can see that I wouldn't substitute anything logically, but I'm using abstract base types. And if set theory is to be believed, then if B extends A, B is still an A. And so, where I can receive an A, I can receive a B. Obviously, I'm missing / lacking an understanding of LSP as it relates to my code. I'm happy to read more, but I'd appreciate some recommendations on what to read. Of course, if anyone can 'correct' my code such that a concrete repo works on concrete entities, but also allows for the base behaviour, then I'd be very grateful. Regards, Richard.