Introduction

Interfaces are definitions of the public APIs classes must implement to satisfy the interface. They work as “contracts”, specifying what a set of subclasses does, but not how they do it.

Interface definition is much alike class definition, changing the keyword class to interface:

interface Foo {

}

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

interface Foo {
    const BAR = 'BAR';

    public function doSomething($param1, $param2);
}

Note: interfaces must not declare constructors or destructors, since these are implementation details on the class level.

Realization

Any class that needs to implement an interface must do so using the implements keyword. To do so, the class needs to provide a implementation for every method declared in the interface, respecting the same signature.

A single class can implement more than one interface at a time.

interface Foo {
    public function doSomething($param1, $param2);
}

interface Bar {
    public function doAnotherThing($param1);
}

class Baz implements Foo, Bar {
    public function doSomething($param1, $param2) {
        // ...
    }

    public function doAnotherThing($param1) {
        // ...
    }
}

When abstract classes implement interfaces, they do not need to implement all methods. Any method not implemented in the base class must then be implemented by the concrete class that extends it:

abstract class AbstractBaz implements Foo, Bar {
    // Partial implementation of the required interface...
    public function doSomething($param1, $param2) {
        // ...
    }
}

class Baz extends AbstractBaz {
    public function doAnotherThing($param1) {
        // ...
    }
}

Notice that interface realization is an inherited characteristic. When extending a class that implements an interface, you do not need to redeclare it in the concrete class, because it is implicit.

Note: Prior to PHP 5.3.9, a class could not implement two interfaces that specified a method with the same name, since it would cause ambiguity. More recent versions of PHP allow this as long as the duplicate methods have the same signature[1].

Inheritance

Like classes, it is possible to establish an inheritance relationship between interfaces, using the same keyword extends. The main difference is that multiple inheritance is allowed for interfaces:

interface Foo {

}

interface Bar {

}

interface Baz extends Foo, Bar {

}

Examples