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

Posted by: admin April 23, 2020 Leave a comment

Questions:

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?

How to&Answers:

How about using static::MY_CONST?

Answer:

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:

Instead of

$c = get_class( $this );
echo $c::MY_CONST;

Do this

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

Answer:

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:

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