PHP multiple classes implementing same interface

Question

If I have an interface in PHP, say

interface AuthenticationInterface {
}

and two classes which implement the interface, for example

class ApiKey implements AuthenticationInterface {
}

class AuthToken implements AuthenticationInterface {
}

How is it determined which of these classes is used when a new AuthenticationInterface in instantiated?

Answer 1

Interfaces are used to define a structure(architecture) for classes that inherit it.

An interface is a contract specifying a set of methods, fields and properties which will be available on any implementing object

I think you were interested in the class factory design pattern. Read more about it here: php design patterns

Answer 2

Interface can't be instantiated. You can create object of class.

Answer 3

If your object was instantiated for you somewhere upstream and all you know is that it implements your AuthenticationInterface interface(so you know it can be one of a small number of classes...2 in your case), you can tell which class your object is by using "instanceof"

if ($your_object instanceof ApiKey)
{
    echo 'object class is ApiKey';
}