PHP: deterring instance variable creation outside the class

Question

In the given PHP class below, I understand that $test will not be accessible from outside this class.

class Resource {
    private $test;
    public function __construct() {
       echo "in init";
       $test = "new val";
    }
}

But we will be able to define, new instance variables as below. Is there a trick to stop this?

$r = new Resource();
$r->val = "value"; 

Answer 1

Using magic methods (namly __set) you can tell the class "if this is not set in here, ignore it", for instance;

<?php

class Resource {
    private $test;
    public function __construct() {
       echo "in init";
       $this->test = "new val";
    }

    public function __set($name, $val)
    {
        // If this variable is in the class, we want to be able to set it
        if (isset($this->{$name})
        {
            $this->{$name} = $val;
        }
        else
        {
            // Do nothing, add an error, anything really
        }
    }
}

$resource = new Resource();
$resource->foo = "bar";
echo $resource->foo; // Will print nothing

For reference, please see the guide