PHP interface example (english)

interface.php

<?php
// interface
interface Color {
	function color(); // define text color
	function bg_color(); // define background color
}
// colors
class Yellow implements Color {
	public function color() {
		return '#000'; // black text
	}
	public function bg_color() {
		return '#fc0'; // yellow background
	}
}
class Red implements Color {
	public function color() {
		return '#fff'; // white text
	}
	public function bg_color() {
		return '#c00'; // red background
	}
}
class Green implements Color {
	public function color() {
		return '#fff'; // white text
	}
	public function bg_color() {
		return '#0c0'; // green background
	}
}
// my classes
class Foo {
	protected $color;
	public function __construct(Color $color) { // notice that I pass the Color interface, not the real color class (Yellow, Red, Green)
		$this->color = $color;
	}
	public function get_color(){
		return $this->color->color();
	}
	public function get_bg_color(){
		return $this->color->bg_color();
	}
}
class Bar {
	protected $color;
	public function __construct(Color $color) { // same as before
		$this->color = $color;
	}
	public function get_color(){
		return $this->color->color();
	}
	public function get_bg_color(){
		return $this->color->bg_color();
	}
}
// let's see what happens...
$color = new Yellow(); // if you change your mind and want the Red or Green background in your classes, you do it once-for-all here!
// create Foo
$foo = new Foo($color);
echo '<p style="color:' . $foo->get_color() . ';background:' . $foo->get_bg_color() . '">I am Foo!<p>';
// create Bar
$bar = new Bar($color);
echo '<p style="color:' . $bar->get_color() . ';background:' . $bar->get_bg_color() . '">I am Bar!<p>';