How to Access Parent Class Variables in Child Classes in PHP

Accessing Parent Class Variables in PHP

When working with inheritance, it becomes necessary to access variables from the parent class. In the provided example, class B extends class A and attempts to echo the variable $bb, which is defined as protected in the parent class. However, an error is thrown, indicating that $bb is an undefined class constant.

To display the parent variable in the child class, use the following syntax:

<code class="php">echo $this->bb;</code>

Unlike private variables, which cannot be accessed outside of the class, protected variables are accessible within child classes. Therefore, $bb can be directly accessed through the $this keyword within the child class. The expected result of 'parent bb' will be printed as expected.

Additional Information: Usage of parent::

The parent:: syntax is used when you want to extend the functionality of a method from the parent class. Unlike the example provided in the question, where the child class merely accesses a parent variable, parent:: allows you to modify or enhance the behavior of an inherited method.

For instance, if the parent class Airplane has a private variable $pilot and a constructor that assigns a pilot to the $pilot variable:

<code class="php">class Airplane {
    private $pilot;

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

And you wish to create a Bomber class that extends Airplane and adds a navigator variable and an extended constructor:

<code class="php">class Bomber extends Airplane {
    private $navigator;

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

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

By using parent::__construct($pilot), you can call the parent class's constructor from the child class, assigning the pilot argument to the $pilot variable in the child class while still adding the navigator functionality. This allows you to reuse existing code from the parent class and customize it in the child class, adhering to the DRY (Don't Repeat Yourself) principle.

The above is the detailed content of How to Access Parent Class Variables in Child Classes in PHP. For more information, please follow other related articles on the PHP Chinese website!