How to work with interfaces

      interface interfaceName {
        const constantName = constantValue;
        public function methodName( parameterList ); 
      }

An interface to show an object

      interface Showable {
        public function show(); 
      }

An interface to require two test methods

      interface Testable {
        public function test1($value1);
        public function test2($value1, $value2); }

An interface that provides three constants

      interface EyeColor {
        const GREEN = 1;
        const BLUE = 2;
        const BROWN = 3; 
      }

A class that inherits a class and implements an interface

      class Employee extends Person implements Showable {
        // The constructor and the get and set methods are the same as the Person class

        Implement the Showable interface 
        public function show() { 
          echo("First Name:" . $this->getFirstName() . "<br />");
          echo("Last Name: " . $this->getLastName() . "<br />");
          echo("SSN: " . $this->ssn . "<br />");
          echo 'Hire Date: ' . $this->hireDate . '"<br />");
        }
      }

A class declaration that implements three interfaces

      class Customer extends Person implements Showable, Testable, EyeColor {
        // class code here
      }

Description

An interface defines a set of public methods that can be implemented by a class. The interface doesn’t provide any code to implement the methods, but it provides the method names and parameter lists.
All methods in an interface must be public and cannot be static.
A class that implements an interface must provide an implementation for each method defined by the interface.
An interface can define class constants that are available to any class that imple-ments the interface.
Although a class can only inherit one class, it can implement multiple interfaces.