How to declare and implement an interface?

We declare an interface with the interface keyword and, the class that inherits from an interface with the implements keyword. Let's see the general case:

interface interfaceName { 
  // abstract methods
}
 
class Child implements interfaceName {
  // defines the interface methods and may have its own code
}

In the simple example given below, we will create an interface for the classes that handle cars, which commits all its child classes to setModel() and getModel() methods.

interface Car { 
  public function setModel($name);
  
  public function getModel();
}

Interfaces, like abstract classes, include abstract methods and constants. However, unlike abstract classes, interfaces can have only public methods, and cannot have variables.

The classes that implement the interfaces must define all the methods that they inherit from the interfaces, including all the parameters. So, in our concrete class with the name of miniCar, we add the code to all the abstract methods.

class miniCar implements Car {
  private $model; 
   
  public function setModel($name)
  { 
    $this -> model = $name; 
  }
  
  public function getModel()
  {
    return $this -> model; 
  }
}

Can we implement more than one interface in the same class?

We can implement a number of interfaces in the same class.

We can implement a number of interfaces in the same class, and so circumvent the law that prohibits the inheritance from more than one parent class. In order to demonstrate multiple inheritance from different interfaces, we create another interface, Vehicle, that commits the classes that implement it to a boolean $hasWheels property.

interface Vehicle {
  public function setHasWheels($bool); 
 
  public function getHasWheels();
}

Now, our child class can implement the two interfaces.

class miniCar implements Car, Vehicle {
  private $model; 
  private $hasWheels; 
  
  public function setModel($name)
  { 
    $this -> model = $name; 
  }
  
  public function getModel()
  {
    return $this -> model; 
  }
  
  public function setHasWheels($bool)
  { 
    $this -> hasWheels = $bool; 
  }
  
  public function getHasWheels()
  {
  return ($this -> hasWheels)? "has wheels" : "no wheels";
  }
}

What are the differences between abstract classes and interfaces?

We saw that abstract classes and interfaces are similar in that they provide abstract methods that must be implemented in the child classes. However, they still have the following differences:

Let's summarize these differences in the following table: