PHP Readonly Properties?

Question

In using PHP's DOM classes (DOMNode, DOMEElement, etc) I have noticed that they possess truly readonly properties. For example, I can read the $nodeName property of a DOMNode, but I cannot write to it (if I do PHP throws a fatal error).

How can I create readonly properties of my own in PHP?

Answer 1

You can't.

Answer 2

You can do it like this:

class Example {
    private $__readOnly = 'hello world';
    function __get($name) {
        if($name === 'readOnly')
            return $this->__readOnly;
        user_error("Invalid property: " . __CLASS__ . "->$name");
    }
    function __set($name, $value) {
        user_error("Can't set property: " . __CLASS__ . "->$name");
    }
}

Only use this when you really need it - it is slower than normal property access. For PHP, it's best to adopt a policy of only using setter methods to change a property from the outside.

Answer 3

Class PropertyExample {

        private $m_value;

        public function Value() {
            $args = func_get_args();
            return $this->getSet($this->m_value, $args);
        }

        protected function _getSet(&$property, $args){
            switch (sizeOf($args)){
             case 0:
              return $property;
             case 1:
              $property = $args[0];
              break; 
             default:
              $backtrace = debug_backtrace();
              throw new Exception($backtrace[2]['function'] . ' accepts either 0 or 1 parameters');
            }
        }


}

This is how I deal with getting/setting my properties, if you want to make Value() readonly ... then you simply just have it do the following instead:

    return $this->m_value;

Where as the function Value() right now would either get or set.

Answer 4

I see you have already got your answer but for the ones who still are looking:

Just declare all "readonly" variables as private or protected and use the magic method __get() like this:

/**
 * This is used to fetch readonly variables, you can not read the registry
 * instance reference through here.
 * 
 * @param string $var
 * @return bool|string|array
 */
public function __get ($var)
{
 return ($var != "instance" && isset($this->$var)) ? $this->$var : false;
}

As you can see I have also protected the $this->instance variable as this method will allow users to read all declared variabled. To block several variables use an array with in_array().

Answer 5

But private properties exposed only using __get() aren't visible to functions that enumerate an object's members - json_encode() for example.

I regularly pass PHP objects to Javascript using json_encode() as it seems to be a good way to pass complex structures with lots of data populated from a database. I have to use public properties in these objects so that this data is populated through to the Javascript that uses it, but this means that those properties have to be public (and therefore run the risk that another programmer not on the same wavelength (or probably myself after a bad night) might modify them directly). If I make them private and use __get() and __set(), then json_encode() doesn't see them.

Wouldn't it be nice to have a "readonly" accessibility keyword?