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Make sense of PHP interfaces

jmau111 profile image Julien Maury Updated on ・2 min read

Indeed PHP interfaces have a purpose.

They are contracts, like instruction manuals for other developers.
However, it might be difficult to understand why interfaces are useful.

Basics

An interface is an abstract class therefore you cannot instantiate it. Instead, you will implement it.

Here is a very basic example:

interface MyInterface {
    public function setName(string $name);
    public function getName();
}

class MyClass implements MyInterface {
    private $name; 

    public function setName(string $name) { 
        $this->name = $name; 
    }

    public function getName() {
        return $this->name; 
    }
}

MyClass must implement setName() and getName(). If you don't do that, you will get a fatal error:

Fatal error: Class MyClass contains 2 abstract methods and must therefore be declared abstract or implement the remaining methods (MyInterface::setName, MyInterface::getName)

Besides, all methods in interfaces must have public visibility scope and interfaces cannot have attributes.

The why

Developers use interfaces to describe how a class or a collection of classes should behave.

Interfaces encapsulate the implementation without handling the details for each class. But why?

Decoupling!

Interfaces allow you to change the implementation without changing the details a.k.a the way you use this implementation.

More advanced example

Any cache would need at least:

  • to store/set something in the cache
  • to get something in the cache
  • to delete something in the cache

For that purpose, we may create the following cache interface:

interface CacheInterface {
    public function set(string $key, $val);
    public function get(string $key);
    public function delete(string $key);
}

This way, we can tell all developers to implement our cache interface without knowing the particular details of each use.

As a result, it's easy to change the caching system without changing the way it's used in the project.

The Symfony example

The previous example makes sense. Let's look at Symfony. When you want to implement any cache system, it's good practice to do the following:

use Symfony\Contracts\Cache\CacheInterface;

class MyClass {
    private $cache;
    public function __construct(CacheInterface $cache)
    {
        $this->cache = $cache;
    }
}

Here we inject the cache implementation into our class (dependency injection). Next time we want to change the cache system, we won't have to modify our class at all.

Multiple implements vs multiple inheritances

PHP doesn’t support multiple inheritances. The following code is not possible:

class MyClass extends ClassX, ClassY {}

This is partly because of the Diamond problem.

Instead, you could do:

class ClassY extends ClassX {}
class MyClass extends ClassY {}

but ClassX and ClassY might handle different things so it does not make any sense to use inheritance.

If you want to enforce multiple behaviors, you can implement multiple interfaces.

class MyClass implements InterfaceX, InterfaceY {}

In other words, you can group classes that share some functionality but do not share a parent class.

Wrap up

PHP interfaces are templates of method signatures for your classes. This is great for decoupling implementation and its use.

It's particularly useful when you need some flexibility while making sure all developers follow a set of rules.

Posted on by:

jmau111 profile

Julien Maury

@jmau111

Practise what you preach.

Discussion

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Editor guide
 

I don't like the suffix "Interface" in its name. Instead, I use the adjective property that the related object must have. This is good for understanding the role of the interface.

 

It depends. Symfony has a lot of Interfaces with that suffix, I guess the most important thing is to keep the same naming convention for consistency.

 

I don't use Symfony, I am not limited by this choice.

For the consistency you may use proxies.

 

You could also use traits.

 

Hi, I agree you can indeed combine the use of traits with interfaces but you cannot use traits instead of interfaces. It depends on what you want to achieve.

 

Traits are a highway to hell.