1. abstract class aaa{} (注意aaa中只有抽象方法，没有一般方法)
class bbb extends aaa{} (在bbb中覆写aaa中的抽象方法)
2. interface aaa{}
class bbb implements aaa{} (在bbb中覆写aaa中的抽象方法)
*/

Listing 6.13 Abstract classes

//abstract root class 抽象根类
abstract class Shape
{
　abstract function getArea(); //定义一个抽象方法
}

//abstract child class 抽象子类
abstract class Polygon extends Shape //多边形
{
　abstract function getNumberOfSides();
}

//concrete class 实体类 三角形类
class Triangle extends Polygon
{
　public $base;
　public $height;

　public function getArea() //覆写计算面积方法
　{
　　return(($this-＞base * $this-＞height)/2);
　}

　public function getNumberOfSides() //覆写边数统计方法
　{
　　return(3);
　}
}

//concrete class 实体类四边形
class Rectangle extends Polygon
{
　public $width;
　public $height;

　public function getArea()
　{
　　return($this-＞width * $this-＞height);
　}

　public function getNumberOfSides()
　{
　　return(4);
　}
}

//concrete class 实体类 圆形
class Circle extends Shape
{
　public $radius;

　public function getArea()
　{
　　return(pi() * $this-＞radius * $this-＞radius);
　}
}

//concrete root class 定义一个颜色类
class Color
{
　public $name;
}

$myCollection = array(); //建立形状的集合，放入数组

//make a rectangle
$r = new Rectangle;
$r-＞width = 5;
$r-＞height = 7;
$myCollection[] = $r;
unset($r);

//make a triangle
$t = new Triangle;
$t-＞base = 4;
$t-＞height = 5;
$myCollection[] = $t;
unset($t);

//make a circle
$c = new Circle;
$c-＞radius = 3;
$myCollection[] = $c;
unset($c);

//make a color
$c = new Color;
$c-＞name = "blue";
$myCollection[] = $c;
unset($c);

foreach($myCollection as $s)
{
　if($s instanceof Shape) //如果$s是Shape类的实例
　{
　　print("Area: " . $s-＞getArea() . "n");
　}

　if($s instanceof Polygon)
　{
　　print("Sides: " .$s-＞getNumberOfSides()."n");
　}

　if($s instanceof Color)
　{
　　print("Color: $s-＞name　n");
　}

　print("n");
}

?＞