文章标题

abstract class Shape{
abstract function length();
}
class Rectangle extends Shape{
private $a;
private $b;
function __construct($a,$b){
$this->a=$a;
$this->b=$b;
}
function length(){
echo "三角形的周长是".$this->a*$this->b;
}
}
$rec =new Rectangle(10,20);
$rec->length();