Is it possible to have an interface that has private / protected methods?

Is it possible in PHP 5 to have an interface that has private / protected methods?

Right now I have:

interface iService
{
    private method1();
}

That throws an error:

> Parse error: syntax error, unexpected T_STRING, expecting T_VARIABLE

I just want to have confirmation that it is the case that an interface can only contain public methods.

The PHP manual page about interfaces explicitly states:

> All methods declared in an interface must be public; this is the nature of an interface.

I guess this explains the error you are getting ;-)

Interfaces are used to describe public methods of a class implementing that interface. You can never have a private method in an interface. Any methods in an interface are assumed to be in use and should not be changed.

Interfaces is the PHP link, but this is standard in OO programming.

In general an interface can only have public members, because the only function of an interface is to be inherited.

From PHPfreaks.com tutorial:

> PHP5 features interfaces. Not to be > confused with interfaces in the more > general sense, the interface keyword > creates an entity that can be used to > enforce a common interface upon > classes without having to extend them > like with abstract classes. Instead an > interface is implemented. > > Interfaces are different from abstract > classes. For one, they’re not actually > classes. They don’t define properties, > and they don’t define any behaviour. > The methods declared in an interface > must be declared in classes that > implement it. > > Because an interface in the more > general sense is a definition of how > an object interacts with other code, > all methods must be declared public > (see section on visibility in this > chapter). Using abstract classes, an > abstract method can have any > visibility, but the extending classes > must have their implementations use > the same (or weaker) visibility. > Implementing an interface adds the > methods as abstract methods to the > subject class, failure to implement it > will result in an error like the > following: > > Fatal error: Class SomeConcreteClass > contains n abstract method(s) and must > therefore be declared abstract or > implement the remaining methodsYes, > abstract classes can implement > interfaces.

interfaces are type declarations. a type is set of values, plus a set of operations that can be carried upon them from outside. a private method doesn't fit into this picture.

interface T {
  public /*int*/ function f(array $a);
}
interface U {
  public /*T*/ function g(T $t);
}

class C implements U {
    public function g(T $t) {
        ...
        $x = $t->f();
        ...
    }
}

interfaces are useful because they state, well, objects' interfaces. how the objects communicate with their environment.

now let's say T::f could be declared private. how would that be useful to other objects? it would not callable from outside, it would not be part of its interface.

In many cases, an interface definition helps other modules guarantee the behavior and the API of a class. In those cases, private methods are not something the other modules can access or understand. That's why you can never put private methods on an interface.

As stated, interfaces can only define the publicly visible methods. I wanted to show an example of how protected methods can be handled. To impose the use of specific protected methods, it is possible to create an abstract class that implements the interface.

This especially makes sense if the abstract class can already handle some of the workload, to simplify the actual implementation. Here for example, an abstract class takes care of instantiating the result object, which is always needed:

First off, the interface.

interface iService
{
   /**
    * The method expects an instance of ServiceResult to be returned.
    * @return ServiceResult
    */
    public function doSomething();
}

The abstract class then defines the internal methods structure:

abstract class AbstractService implements iService
{
    public function doSomething()
    {
        // prepare the result instance, so extending classes
        // do not have to do it manually themselves.
        $result = new ServiceResult();

        $this->process($result);

        return $result;
    }

   /**
    * Force all classes that extend this to implement
    * this method.
    *
    * @param ServiceResult $result
    */
    abstract protected function process($result);
}

The class that does the actual implementation automatically inherits the interface from the abstact class, and only needs to implement the protected method.

class ExampleService extends AbstractService
{
    protected function process($result)
    {
         $result->setSuccess('All done');
    }
}

This way the interface fulfills the public contract, and through the AbstractService class, the internal contract is fulfilled. The application only needs to enforce the use of the AbstractService class wherever applicable.

Big NO, any method in the Interface will never have private or protected access identifier.

**All methods declared in an interface must be public; this is the nature of an interface.

Few other interesting facts about interface

Interfaces can be extended like classes using the extends operator. They can extend only other interfaces. (source: https://www.php.net/manual/en/language.oop5.interfaces.php)

Note that it is possible to declare a constructor in an interface, which can be useful in some contexts, e.g. for use by factories. Signature should be same in the child class.

In your case, even another problem is - function keyword is missing in the function declaration. It should be

interface iService
{
    public function method1();
}