Why do I have to declare __set if I declare __get for read-only properties?

Howdy, I'd like to access some of the private members of my classes as read-only properties without resorting to function calls to access them. (e.g. $testClass->privateMember instead of $testClass->privateMember(), etc) Based on my research and testing, using the __get and __set overloading methods appears to be the only way to do so. It also, based on testing, appears that these private members must be in an array. What I do not understand is that if I declare a __get method I MUST also declare a "do nothing" __set method to prevent the read-only properties from being modified in code that uses the class. For example, the code below allows me to have read-only properties. However, if I remove the "do nothing" __set method completely, then the properties are no longer read-only. I'm curious as to why I HAVE to implement the __set method? Example: <?php class testClass { private $varArray = array('one'=>'ONE', 'two'=>'TWO'); public function __get($name) {

       if (array_key_exists($name, $this->varArray)) {
           return $this->varArray[$name];
       }

} public function __set($name, $value) { } } $test = new testClass(); $test->one = 'TWO'; // doesn't work

echo $test->one;      // echo s 'ONE'

?>