How to access constant defined in child class from parent class functions? - php

Question

I saw this example from php.net:
<?php
class MyClass {
const MY_CONST = "yonder";
public function __construct() {
$c = get_class( $this );
echo $c::MY_CONST;
}
}
class ChildClass extends MyClass {
const MY_CONST = "bar";
}
$x = new ChildClass(); // prints 'bar'
$y = new MyClass(); // prints 'yonder'
?>
But $c::MY_CONST is only recognized in version 5.3.0 or later. The class I'm writing may be distributed a lot.
Basically, I have defined a constant in ChildClass and one of the functions in MyClass (father class) needs to use the constant. Any idea?

Answer 1

How about using static::MY_CONST?

Answer 2

Since php 5.3:
Use static::MY_CONST
More details on static
In this case the keyword static is a reference to the actually called class.
This illustrates the difference between static $var, static::$var and self::$var:
class Base {
const VALUE = 'base';
static function testSelf() {
// Output is always 'base', because `self::` is always class Base
return self::VALUE;
}
static function testStatic() {
// Output is variable: `static::` is a reference to the called class.
return static::VALUE;
}
}
class Child extends Base {
const VALUE = 'child';
}
echo Base::testStatic(); // output: base
echo Base::testSelf(); // output: base
echo Child::testStatic(); // output: child
echo Child::testSelf(); // output: base
Also note that the keyword static has 2 quite different meanings:
class StaticDemo {
static function demo() {
// Type 1: `static` defines a static variable.
static $Var = 'bar';
// Type 2: `static::` is a reference to the called class.
return static::VALUE;
}
}

Answer 3

Instead of
$c = get_class( $this );
echo $c::MY_CONST;
Do this
$c = get_class( $this );
echo constant($c . '::MY_CONST');

Answer 4

I couldn't get it to work with const as it prints "yonderyonder" (that's the thing about constants, they don't change), but it works fine with var:
<?php
class MyClass {
var $MY_CONST = "yonder";
public function __construct() {
echo $this->MY_CONST;
}
}
class ChildClass extends MyClass {
var $MY_CONST = "bar";
}
$x = new ChildClass(); // prints 'bar'
$y = new MyClass(); // prints 'yonder'
?>

Answer 5

If you need to access constants, properties, methods of classes or objects you can use reflection, it provides much more details about structure of the object.
example:
class MainClass
{
const name = 'Primary';
public $foo = 'Foo Variable';
}
class ExtendedClass extends MainClass
{
const name = 'Extended';
}
/**
* From Class Name
*/
//get reflection of main class
$mainReflection = new ReflectionClass('MainClass');
if($mainReflection->hasConstant('name'))
var_dump($mainReflection->getConstant('name'));//Primary
//get reflection of extended class
$extendedReflection = new ReflectionClass('ExtendedClass');
if($extendedReflection->hasConstant('name'))
var_dump($extendedReflection->getConstant('name'));//Extended
/**
* From Objects
*/
$main = new MainClass();
$extended = new ExtendedClass();
//get reflection of main class
$mainReflection = new ReflectionObject($main);
if($mainReflection->hasConstant('name'))
var_dump($mainReflection->getConstant('name'));//Primary
//get reflection of extended class
$extendedReflection = new ReflectionObject($extended);
if($extendedReflection->hasConstant('name'))
var_dump($extendedReflection->getConstant('name'));//Extended