面向例题

例子：求两个圆之间的阴影面积
/*$maxr = 20;
$minr = 10;
$mj = 3.14*$maxr*$maxr - 3.14*$minr*$minr;

class Yuan
{
//代表半径
public $r;

function __construct($n)
{
$this->r = $n;
}

//求面积的方法
function MJ()
{
return 3.14*$this->r*$this->r;
}
}

$r1 = new Yuan(20);
$r2 = new Yuan(10);

$mianji = $r1->MJ()-$r2->MJ();