Abstract constants in PHP – Force a child class to define a constant

I noticed that you can’t have abstract constants in PHP.

Is there a way I can force a child class to define a constant (which I need to use in one of the abstract class internal methods) ?

Answer

A constant is a constant; there is no abstract or private constants in PHP as far as I know, but you can have a work around:

Sample Abstract Class

abstract class Hello {
    const CONSTANT_1 = 'abstract'; // Make Abstract
    const CONSTANT_2 = 'abstract'; // Make Abstract
    const CONSTANT_3 = 'Hello World'; // Normal Constant
    function __construct() {
        Enforcer::__add(__CLASS__, get_called_class());
    }
}

This would run fine

class Foo extends Hello {
    const CONSTANT_1 = 'HELLO_A';
    const CONSTANT_2 = 'HELLO_B';
}
new Foo();

Bar would return Error

class Bar extends Hello {
    const CONSTANT_1 = 'BAR_A';
}
new Bar();

Songo would return Error

class Songo extends Hello {

}
new Songo();

Enforcer Class

class Enforcer {
    public static function __add($class, $c) {
        $reflection = new ReflectionClass($class);
        $constantsForced = $reflection->getConstants();
        foreach ($constantsForced as $constant => $value) {
            if (constant("$c::$constant") == "abstract") {
                throw new Exception("Undefined $constant in " . (string) $c);
            }
        }
    }
}