PHP Accessing Parent Class Variable

Question

class A {
    private $aa;
    protected $bb = 'parent bb';

    function __construct($arg) {
       //do something..
    }

    private function parentmethod($arg2) {
       //do something..
    }
}

class B extends A {
    function __construct($arg) {
        parent::__construct($arg);
    }
    function childfunction() {
        echo parent::$bb; //Fatal error: Undefined class constant 'bb' 
    }
}

$test = new B($some);
$test->childfunction();

Question: How do I display parent variable in child? expected result will echo 'parent bb'

Solution 1

echo $this->bb;

The variable is inherited and is not private, so it is a part of the current object.

---

Here is additional information in response to your request for more information about using parent:::

Use parent:: when you want add extra functionality to a method from the parent class. For example, imagine an Airplane class:

class Airplane {
    private $pilot;

    public function __construct( $pilot ) {
        $this->pilot = $pilot;
    }
}

Now suppose we want to create a new type of Airplane that also has a navigator. You can extend the __construct() method to add the new functionality, but still make use of the functionality offered by the parent:

class Bomber extends Airplane {
    private $navigator;

    public function __construct( $pilot, $navigator ) {
        $this->navigator = $navigator;

        parent::__construct( $pilot ); // Assigns $pilot to $this->pilot
    }
}

In this way, you can follow the DRY principle of development but still provide all of the functionality you desire.

Solution 2

Just echo it since it's inherited

echo $this->bb;

Solution 3

With parent::$bb; you try to retrieve the static constant defined with the value of $bb.

Instead, do:

echo $this->bb;

Note: you don't need to call parent::_construct if B is the only class that calls it. Simply don't declare __construct in B class.

Solution 4

$bb has now become the member of class B after extending class A.

So you access $bb like it's an attribute of class B.

class A {
    private $aa;
    protected $bb = 'parent bb';

    function __construct($arg) {
       //do something..
    }

    private function parentmethod($arg2) {
       //do something..
    }
}

class B extends A {
    function __construct($arg) {
        parent::__construct($arg);
    }
    function childfunction() {
        echo $this->bb; 
    }
}

$test = new B($some);
$test->childfunction();

Solution 5

class A {
    private $aa;
    protected $bb = 'parent bb';

    function __construct($arg) {
       //do something..
    }

    private function parentmethod($arg2) {
       //do something..
    }
}

class B extends A {
    function __construct($arg) {
        parent::__construct($arg);
    }
    function childfunction() {
        echo parent::$this->bb; //works by M
    }
}

$test = new B($some);
$test->childfunction();`

View more solutions

Comments

[Kuntau]

so thats mean parent keyword only used to access parent method?

[George Cummins]

Usually, you would use parent:: when you want to override a parent method, but still reference the parent's functionality. If you just want to call the parent's method, you do it the same way as for a variable: $this->parentmethod()

[Tobia]

parent::$this->bb has got to be the craziest object-oriented syntax I've ever seen

[T30]

protected doesn't throw errors when used in inherited and parent classes.

[SEoF]

parent:: in parent::$this->bb achieves nothing but confusing the person asking. I recommend removal or further explanation in order to reduce complexity of the answer.

[KD.S.T.]

this doesn't work Fatal error: Uncaught Error: Access to undeclared static property: A::$this in /Applications/MAMP/htdocs/native_testing/index.php:24 Stack trace: #0 /Applications/MAMP/htdocs/native_testing/index.php(29): B->childfunction() #1 {main} thrown in /Applications/MAMP/htdocs/native_testing/index.php on line 24

[MilanG]

Just make sure that variable is not defined as private.

[user2924019]

I cannot get this to work. The variable is public, but the variables just aren't getting inherited.

[user2924019]

Apparently it's not. I assumed it would be but found they don't get inherited.

[user2924019]

This code returns Fatal error: Access to undeclared static property: A::$this

[Timothy Alexis Vass]

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