PHP类可以返回它们正在实现的类型接口的对象吗？

Question

When concreate classes implement interface, do they return object of type interface that they are implementing? In typical factory pattern scenario the concrete classes return object of type interface they are implementing (for java and C# this is true). Does this happen in case of PHP also?

Consider the following situation..

interface IUser
{
  function getName();
}

class User implements IUser
{
  public function __construct( $id ) { }

  public function getName()
  {
    return "Jack";
  }
}

class UserFactory
{
  public static function Create( $id )
  {
    return new User( $id );
  }
}

$uo = UserFactory::Create( 1 );
echo( $uo->getName()."
" );

Is object $uo an instance of type IUser or User, and why?

Answer 1

You never return an interface or a class, you return an object that implements the class of your choice which in turn will implement an interface.

An interface or class is the declaration and definition portion of the object that you create while the object is variable that contains the properties. Any call you do to an object resolves to a class and the body of that function is executed, but the properties you access (the non-static ones) return to the object itself.

When instanciating an object of a certain class, the object is OF that class. It cannot be of the type of an interface because an interface cannot be instanciated. On the other hand, you can use "instanceof" in PHP to determine if an object implements a certain interface or uses a specific class. Instanceof validates all interfaces implemented in all classes of the object, even the parent ones.

So using:

interface i {}
interface i2 {}
class a implements i {}
class b extends a {}
class c extends b implements i2 {}
$o = new b();

In this very case, $o is an "i", "a" and "b". All calls to instanceof with these class names will return true... But because $o is ob class b naturally, instanceof will never return true for the C class or the i2 interface.