Why interfaces are used in php?

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Interface are definitions of the public APIs that classes (implementing an interface) must implement. It is also known as contracts as an interface allows to specify a list of methods that a class must implement. Interface definition is similar to the class definition, just by changing the keyword class to interface.
Example:
 

php

Interfaces can contain methods and/or constants, but no attributes. Interface constants have the same restrictions as class constants. Interface methods are implicitly abstract.
Example:
 

php

interface IMyInterface {

    const INTERFACE_CONST_1 = 1;

    const INTERFACE_CONST_2 = 'a string';

    public function method_1();

    public function method_2();

}

?>

Any class that needs to implement an interface must do using the implements keyword. A single class can implement more than one interface at a time.
 

php

class MyClass implements IMyInterface {

    public function method_1() {

    }

    public function method_2() {

    }

}

Interesting Points : 

• A class cannot implement two interfaces that has the a same method name because it would end up with the method ambiguity.
• Like classes, it is possible to establish an inheritance relationship between interfaces by using the same keyword “extends”. 
Example: 
 

php

interface Foo {

}

interface Bar {

}

interface Bass extends Foo, Bar {

}

Below is a complete example that shows how interfaces work. 
Example: 
 

php

interface Shape {

    public function getColor();

    public function setColor($color);   

    public function describe();

}

class Triangle implements Shape {

    private $color = null;

    public function getColor() {

        return $this->color;

    }  

    public function setColor($color) {

        $this->color = $color;

    }  

    public function describe() {

        return sprintf("GeeksforGeeks %s %s\n",

            $this->getColor(), get_class($this));

    }  

}

$triangle = new Triangle();

$triangle->setColor('green');

print $triangle->describe();

?>

Output: 
 

GeeksforGeeks green Triangle

Importance of using Interfaces: 
 

• Interface provide a flexible base/root structure that you don’t get with classes.
• By implementing interface the caller of the object need to care only about the object’s interface not implementations of the object’s methods.
• Interface allows unrelated classes to implement the same set of methods regardless of their positions in the class inheritance hierarchy.
• Interface enables you to model multiple inheritance.