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phpoopoverridingclass-constants

Using class constants and overriding in PHP

If I have a class structure with a value that can either be true or false, that doesn't change, currently implemented as variables would it be better to change them to constants, such as:

class Parent {
    const BOOL_CONST = false;

    ...
}

class SomeChild extends Parent {
    const BOOL_CONST = true;

    ...
}

Later I have an object which may be of any type in that class hierarchy, either the parent or one of its children, and some of the children may, like 'SomeChild' have overloaded the value to be true.

Is there some way I can access the constant without knowing the class? In other words can I do something like:

$object->BOOL_CONST

Or would it be better to leave these values as variables, even though they really shouldn't change?

UPDATE

I've reworded my question above to better express what I was attempting to ask.

Solution

  • PHP 5.3 now accepts the object as the class reference: $this::BOOL_CONST is now accepted.

    //
// http://php.net/manual/en/language.oop5.constants.php
//
// As of PHP 5.3.0, it's possible to
// reference the class using a variable.
// The variable's value can not be a keyword
// (e.g. self, parent and static). 
//

// I renamed "Parent" class name to "constantes"
// because the classname "Parent" can be confused with "parent::" scope
class constantes
{
    const  test                     = false;
}

// I renamed "SomeChild" too, with no reason...
class OverloadConst extends constantes
{
    const test                      = true;
    public function waysToGetTheConstant()
    {
        var_dump(array('$this'=>$this::test));    // true, also usable outside the class
        var_dump(array('self::'=>self::test));    // true, only usable inside the class
        var_dump(array('parent::'=>parent::test));    // false, only usable inside the class
        var_dump(array('static::'=>static::test));    // true, should be in class's static methods, see http://php.net/manual/en/language.oop5.late-static-bindings.php
    }
}

// Classic way: use the class name
var_dump(array('Using classname'    => OverloadConst::test));

// PHP 5.3 way: use the object
$object = new OverloadConst();
var_dump(array('Using object'       => $object::test));
$object->waysToGetTheConstant();

    Note that you can override a class constant, but not an interface constant. If constantes is an interface that OverloadConsts implements, then you can not override its const test (or BOOL_CONST).

    Sources