Practicing Loose'ly Coupling code in php - php

Question

I'm new to PHP or any progamming language and im practic'n how to code a loose coupling
<?php
interface IPolygon{
public function getArea();
}
class Polygon{
private $ipolygon;
public function __construct(IPolygon $ipolygon){
$this->ipolygon = $ipolygon;
}
public function calArea(){
return this->ipolygon->getArea();
}
}
class Circle implements IPolygon{
private $radius;
private $pi = 3.14;
public function __construct($radius){
$this->radius = $radius;
}
public function getArea(){
return $pi*$radius*$radius;
}
}
class Triangle implements IPolygon{
private $base;
private $height;
public function __construct($base , $height){
$this->base = $base;
$this->height = $height;
}
public function getArea(){
return $base*$height/2;
}
}
$c = new Circle(14);
$p = new Polygon($c);
echo "<hr />";
$p->calArea();
?>
Error
Notice: Undefined variable: ipolygon in C:\xampp\htdocs\OOP\Polygon.php on line 16
Notice: Undefined variable: pi in C:\xampp\htdocs\OOP\Polygon.php on line 30
Notice: Undefined variable: radius in C:\xampp\htdocs\OOP\Polygon.php on line 30
Notice: Undefined variable: radius in C:\xampp\htdocs\OOP\Polygon.php on line 30
can someone explain me how is it done?

Answer 1

As a general rule all class properties are referenced using $this->
That means:
<?php
interface IPolygon{
public function getArea();
}
class Polygon{
private $ipolygon;
public function __construct(IPolygon $ipolygon){
$this->ipolygon = $ipolygon;
}
public function calArea(){
return $this->ipolygon->getArea();
}
}
class Circle implements IPolygon{
private $radius;
private $pi = 3.14;
public function __construct($radius){
$this->radius = $radius;
}
public function getArea(){
return $this->pi*$this->radius*$this->radius;
}
}
class Triangle implements IPolygon{
private $base;
private $height;
public function __construct($base , $height){
$this->base = $base;
$this->height = $height;
}
public function getArea(){
return $this->base*$this->height/2;
}
}
$c = new Circle(14);
$p = new Polygon($c);
echo "<hr />";
$p->calArea();
It works: http://sandbox.onlinephpfunctions.com/code/e7f79bb186ffac952ebc4c2b5a8098e1efaeb874