How to make a final variable in PHP?

Question

I can't find out, or maybe I am thinking wrongly but I need to make a variable that can't be changed, like read-only, something like :

final $finalVar = 'extremely secret number'; // don't change

$finalVar = 'hacked...'; // THROW I GIANT BIG ERROR HERE !

Answer 1

Aside from constants (as mentioned in comments), the only way I can think of to do this is to use a parent-child relationship with a private variable

class ParentC {
    private $var = 'bob';
}

class ChildC extends ParentC {
    public function setVar() {
         // Fatal error: Uncaught Error: Cannot access private property ParentC::$var
         echo parent::$var; 
    }
}

Note that there's a hacky way around that using the Reflection class. But, for the most part, you can't touch a private parent variable from a child class

Answer 2

While there has been talk of read-only variables since at least 2012, with even an RFC proposing it on objects, the support does not exist in the language.

One way to achieve a read-only variable (or a collection of read-only variables, as might be important with certain configuration values) is with a mediating container:

class Readonly {
    public function __construct(...$vars) {
        $this->vars;
    }

    public function __set($var, $value) {
        if (array_key_exists($var, $this->vars)) {
            throw new \LogicException("Variable $var is read-only");
        } else {
            $this->vars[$var] = $value;
        }
    }

    public function __get($var) {
        return array_key_exists($var, $this->vars) ? $this->vars[$var] : null;
    }

    protected $vars = [];
}

Which allows you to create a container of read-only variables:

$config = new Readonly('apikey');

$config->apikey = 'A01AB020'; // this works, first time set
echo $config->apikey;

$config->apikey = '00000000'; // boom! it's "final"

Answer 3

With PHP 8.1 you can now declare a variable as readonly :

class MyClass{
    public readonly string $prop;
 
    public function __construct(string $val) {
        // Can be intialized only once.
        $this->prop = $val;
    }
}

$myclass = new MyClass('Foo');
$myclass->prop; // Read the property
$myclass->prop = 'Bar'; // Error: Cannot modify readonly property

Note that you can only apply readonly to typed properties.

Answer 4

Use constant:

defined('VARIABLE')  OR define('VARIABLE', 'value');

Documentation: define defined

Answer 5

You can use constants if you want to create variables which you don't want to be changed:

class MyClass {

   const VERSION = '2.1'; // This constant can be view outside the class,
                          // but its value can't be changed even in this class

   function myMethod () {
       echo self::VERSION; // Inside class
   }

}

or outside the class:

echo MyClass::VERSION;

Functional approach:

define ('VERSION', '2.1');

echo VERSION;