The Liskov substitution principle talks about inheritance. It specifies how we should design our classes so that client dependencies can be replaced by subclasses without the client seeing the difference, as per the definition found on Wikipedia:
"objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program"
While there might be some specific functionality added to the subclass, it has to conform to the same behavior as its base class. Otherwise the Liskov principle is violated.
When it comes to PHP and sub-classing, we have to look beyond simple concrete classes and differentiate: concrete class, abstract class, and interface. Each of the three can be put in the context of a base class, while everything extending or implementing it can be looked at as a derived class.
The following is an example of LSP violation, where the derived class does not have an implementation for all methods:
interface User...