Declaration of Methods should be Compatible with Parent Methods in PHP

In PHP, when a class inherits from another class, it is said to be a subclass or derived class. The subclass inherits all of the properties and methods of the parent class, but it can also add or override those methods.

When a subclass overrides a method of the parent class, it must have the same method signature. This means that the method name and the number and type of parameters must be the same. The return type can be different, but it should be compatible with the return type of the parent method. This is known as method compatibility or method signature compatibility.

For example, if a parent class has a method named "calculate" that takes two integers as parameters and returns an integer, a subclass that overrides that method must also have a method named "calculate" that takes two integers as parameters and returns a value that is compatible with an integer.

By following these rules, the subclass can be used interchangeably with the parent class, and the parent class' methods can be called on an object of the subclass without causing errors.