按照书上的写了个计算图形面积的程序，但是不执行计算，请问哪里出错了
《细说php》第8章的最后一个例子。可以创建表单，但是点击计算之后出错误。请各位大侠指点迷津
接口：class__Shape.php
interface Shape{
function area();
function perimeter();
}
?>
矩形：class__Rect.php
class Rect implements Shape{
private $width;
private $length;
function __construct($size=""){
$this->width=$size["width"];
$this->length=$size["length"];
}
function area(){
return $this->length * $this->width;
}
function perimeter(){
return 2 * ($this->width + $this->length);
}
}
?>
三角形：class__Triangle.php
class Triangle implements Shape{
private $length1;
private $length2;
private $length3;
function __construct($size=""){
$this->length1 = $size["length1"];
$this->length2 = $size["length2"];
$this->length3 = $size["length3"];
}
function area(){
$s = ($this->length1 + $this->length2 + $this->length3)/2;
return sqrt($s * ($s - $this->length1) * ($s - $this->length2) * ($s - $this->length3));
}
function perimeter(){
return $this->length1 + $this->length2 + $this->length3;
}
}
?>
圆形：class__Circle.php
class Circle implements Shape{
private $radius;
function __construce($size=""){
$this->radius = $size["radius"];
}
function area(){
return pi() * $this->radius * $this->radius;
}
function perimeter(){
return 2* pi() * $this->radius;
}
}
?>
表格：class__Form.php
class Form{
private $formName;
private $request;
private $action;
private $method;
private $target;
function __construct($formName,$request,$action="index.php",$method="get",$target="_self"){
$this->formName=$formName;
$this->request=$request;
$this->action=$action;
$this->method=$method;
$this->target=$target;
}
function __toString(){
$str="
