How to implement a read-only member variable in PHP?

I suppose a solution, for class properties, would be to :

• not define a property with the name that interests you
• use the magic __get method to access that property, using the "fake" name
• define the __set method so it throws an exception when trying to set that property.
• See Overloading, for more informations on magic methods.

For variables, I don't think it's possible to have a read-only variable for which PHP will throw an exception when you're trying to write to it.

For instance, consider this little class :

class MyClass {
    protected $_data = array(
        'myVar' => 'test'
    );

    public function __get($name) {
        if (isset($this->_data[$name])) {
            return $this->_data[$name];
        } else {
            // non-existant property
            // => up to you to decide what to do
        }
    }

    public function __set($name, $value) {
        if ($name === 'myVar') {
            throw new Exception("not allowed : $name");
        } else {
            // => up to you to decide what to do
        }
    }
}

Instanciating the class and trying to read the property :

$a = new MyClass();
echo $a->myVar . '<br />';

Will get you the expected output :

test

While trying to write to the property :

$a->myVar = 10;

Will get you an Exception :

Exception: not allowed : myVar in /.../temp.php on line 19

class test {
   const CANT_CHANGE_ME = 1;
}

and you refer it as test::CANT_CHANGE_ME