我是php新手。我想添加两个数字，但代码有问题......

<?php
class add
{

private $num1;  
private $num2;

public function set($property,$value)
{
	$this->$property=$value;
}

public function get($property)
{
	return $this->$property;
}

}//end of class math

$obj = new add;
$obj -> set("num1",15);
$obj -> set("num2",3);
echo "First_Number = ".$obj->get("num1")."<br>" ; 
echo "Second_Number = ".$obj->get("num2")."<br>" ;
echo "Addition = " .$obj->get("num1") + $obj->get("num2");
?>

我有什么尝试过：



使用相同的代码.....乘法和除法运算正在运行，但加法和减法有问题...... .. ?????

What I have tried:

With the same code.....Multiplication and Division operation are working but having problem with addition and subtraction........?????