PHP在课堂上添加两个数字

I'm trying to build a function inside a PHP class, however whenever I invoke the function, I am only returning the first variable.

        class Nums  
{
    private $a = 7;
    private $b = 8;

    public function sum() 
    {
        return $this->a + $this->b;
    }

    public function __set($name,$value) {
      switch($name) { 
        case 'a': 
          return $this->setA($value);
        case 'b': 
          return $this->setB($value);
      }
    }

    public function __get($name) {
      switch($name) {
        case 'a': 
          return $this->getA();
        case 'b': 
          return $this->getB();
      }
    }

    private function setA($i) {
      $this->a = $i;
    }

    private function getA() {
      return $this->$a;
    }

    private function setB($i) {
      $this->b = $i;
    }

    private function getB() {
      return $this->$b;
    }

}

Am I doing something wrong here, because I can't really see what is wrong with this logic.

  class Nums  
  {
    private $a = 7;
    private $b = 8;

    public function sum() 
    {
        return $this->a + $this->b;
    }
  }

$numObj = new Nums();
echo $numObj->sum();

Running this code returns 15 for me

It's working for me. Here's what i tried and it output 15.

PHP CODE :

<?php

class Nums  
{
    private $a = 7;
    private $b = 8;

    public function sum() 
    {
        return $this->a + $this->b;
    }

}

$obj = new Nums();
$c = $obj->sum();
echo $c;

?>

OUTPUT :

15

<?php

class A
{
    private $a = 7;
    private $b = 8;

    public function sum() 
    {
        return $this->a + $this->b;
    }

}

$obj = new A();
$c = $obj->sum();
echo $c;

?>