PHP Readonly Properties?

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?

Solution 1:

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.

Solution 2:

Since PHP 8.1 there are implemented native readonly properties

Documentation

You can initialize readonly property only once during the declaration of the property.

class Test {
    public readonly string $prop;

    public function __construct(string $prop) {
        $this->prop = $prop;
    }
}

--

class Test {
    public function __construct(
        public readonly string $prop,
    ) {}
}

Trying to modify the readonly propety will cause following error:

Error: Cannot modify readonly property Test::$prop