Inheritance In a PHP Class and Object-Oriented Programming

Let’s talk about inheritance in PHP. First, here’s how you do it:

<?php
class Daddy {
    public function my_method() {
        echo "I'm the parent";
    }
}

class Son extends Daddy {
    
}

$child = new Son;
$child->my_method(); // This outputs "I'm the parent" from the parent class

The child is the class DOING the extending. The parent is the class BEING extended. And, the child inherits any public or protected properties and methods from the parent. This is why “Son” has no methods, but still outputs the results of my_method(). Because it inherited that method from Daddy.

But, private methods and properties are NOT inherited. So, this won’t work:

<?php
class Daddy {
    public function my_method() {
        echo "I'm the parent";
    }
    
    private function you_cant_haz() {
        echo "Only Daddy gets me!";
    }
}

class Son extends Daddy {
    
}

$child = new Son;
$child->you_cant_haz(); // This triggers a fatal error

You can also override inherited methods in the child. Like this:

<?php
class Daddy {
    public $var = "Call me Daddy! ";
    
    public function my_method() {
        echo $this->var;
    }
    
    private function you_cant_haz() {
        echo "Only Daddy gets me!";
    }
}

class Son extends Daddy {
    public $var = "Uh, that's creepy. No!";
    
    public function my_method() {
        echo $this->var;
    }
}

$parent = new Daddy;
$parent->my_method(); // This outputs "Call me Daddy!"

$child = new Son;
$child->my_method(); // This outputs "Uh, that's creepy. No!"

One example use case for this is WordPress widgets. When creating a new widget, your individual widget class extends WP_Widget to access and override its default methods. WordPress then does the “magic” of putting your widget together simply by you overriding the WP_Widget methods in your class.

Another example is the example OOP application shell I show you how to build in my object-oriented programming course. I give you a primer on MVC, which is obviously super popular right now, but is only one among many design patterns. But, I also show you another pattern I learned a few years back that I personally like better… and it involves some strategic inheritance like this. You can learn more about and enroll in my object-oriented programming course here.

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