http://php.net/manual/en/language.oop5.interfaces.php
http://php.net/manual/zh/language.oop5.interfaces.php
Object Interfaces
Object interfaces allow you to create code which specifies which methods a class must implement, without having to define how these methods are handled.
Interfaces are defined in the same way as a class, but with the interface keyword replacing the class keyword and without any of the methods having their contents defined.
All methods declared in an interface must be public; this is the nature of an interface.
implements
To implement an interface, the implements operator is used. All methods in the interface must be implemented within a class; failure to do so will result in a fatal error. Classes may implement more than one interface if desired by separating each interface with a comma.
Note:
Interfaces can be extended like classes using the extends operator.
Note:
The class implementing the interface must use the exact same method signatures as are defined in the interface. Not doing so will result in a fatal error.
Constants
It's possible for interfaces to have constants. Interface constants works exactly like class constants except they cannot be overridden by a class/interface that inherits them.
对象接口
使用接口（interface），可以指定某个类必须实现哪些方法，但不需要定义这些方法的具体内容。
接口是通过 interface 关键字来定义的，就像定义一个标准的类一样，但其中定义所有的方法都是空的。
接口中定义的所有方法都必须是公有，这是接口的特性。
实现（implements）
要实现一个接口，使用 implements 操作符。类中必须实现接口中定义的所有方法，否则会报一个致命错误。类可以实现多个接口，用逗号来分隔多个接口的名称。
Note:
实现多个接口时，接口中的方法不能有重名。
Note:
接口也可以继承，通过使用 extends 操作符。
Note:
类要实现接口，必须使用和接口中所定义的方法完全一致的方式。否则会导致致命错误。
常量
接口中也可以定义常量。接口常量和类常量的使用完全相同，但是不能被子类或子接口所覆盖。
An interface, together with type-hinting, provides a good way to make sure that a particular object contains particular methods. See instanceof operator and type hinting.
接口加上类型约束，提供了一种很好的方式来确保某个对象包含有某些方法。参见 instanceof 操作符和类型约束。
What Is an Interface? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts) https://docs.oracle.com/javase/tutorial/java/concepts/interface.html
Interface in Java - Javatpoint https://www.javatpoint.com/interface-in-java
Java Interfaces http://tutorials.jenkov.com/java/interfaces.html