Why would you need an interface, if there are already abstract classes?
To prevent multiple inheritance (can cause multiple known problems).
One of such problems:
The "diamond problem" (sometimes referred to as the "deadly diamond of
death") is an ambiguity that arises when two classes B and C inherit
from A and class D inherits from both B and C. If there is a method
in A that B and C have overridden, and D does not override it, then
which version of the method does D inherit: that of B, or that of C?
Source: https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem
Why/When to use an interface?
An example... All cars in the world have the same interface (methods)... AccelerationPedalIsOnTheRight(), BrakePedalISOnTheLeft(). Imagine that each car brand would have these "methods" different from another brand. BMW would have The brakes on the right side, and Honda would have brakes on the left side of the wheel. People would have to learn how these "methods" work every time they would buy a different brand of car. That's why it's a good idea to have the same interface in multiple "places."
What does an interface do for you (why would someone even use one)?
An interface prevents you from making "mistakes" (it assures you that all classes which implement a specific interface, will all have the methods which are in the interface).
// Methods inside this interface must be implemented in all classes which implement this interface.
interface IPersonService
{
public function Create($personObject);
}
class MySqlPerson implements IPersonService
{
public function Create($personObject)
{
// Create a new person in MySql database.
}
}
class MongoPerson implements IPersonService
{
public function Create($personObject)
{
// Mongo database creates a new person differently then MySQL does. But the code outside of this method doesn't care how a person will be added to the database, all it has to know is that the method Create() has 1 parameter (the person object).
}
}
This way, the Create() method will always be used the same way. It doesn't matter if we are using the MySqlPerson class or the MongoPerson class. The way how we are using a method stays the same (the interface stays the same).
For example, it will be used like this (everywhere in our code):
new MySqlPerson()->Create($personObject);
new MongoPerson()->Create($personObject);
This way, something like this can't happen:
new MySqlPerson()->Create($personObject)
new MongoPerson()->Create($personsName, $personsAge);
It's much easier to remember one interface and use the same one everywhere, than multiple different ones.
This way, the inside of the Create() method can be different for different classes, without affecting the "outside" code, which calls this method. All the outside code has to know is that the method Create() has 1 parameter ($personObject), because that's how the outside code will use/call the method. The outside code doesn't care what's happening inside the method; it only has to know how to use/call it.
You can do this without an interface as well, but if you use an interface, it's "safer" (because it prevents you to make mistakes). The interface assures you that the method Create() will have the same signature (same types and a same number of parameters) in all classes that implement the interface. This way you can be sure that ANY class which implements the IPersonService interface, will have the method Create() (in this example) and will need only 1 parameter ($personObject) to get called/used.
A class that implements an interface must implement all methods, which the interface does/has.
I hope that I didn't repeat myself too much.
