<?php
require_once("Dog.php");

class GermanShepherd extends Dog
{

    static int $num_of_dogs_created = 0;

    public static function update_num_of_dogs_created() {
        self::$num_of_dogs_created++;
    }
  
    //...
}

What we’ve done here is creating a static property called $num_of_dogs_created and below that a static method called update_num_of_dogs_created(). These are both class members and to access them you do not have to instantiate the GermanShepherd object.

To test our example, we’ll create a new file and add the same code we did in the previous article.

<?php

require_once("GermanShepherd.php");
$gs_dog = new GermanShepherd("Brown", "10/10/2019", "Brown");

echo "Num of instances created: " . GermanShepherd::$num_of_dogs_created . "<br>";

GermanShepherd::update_num_of_dogs_created();
echo "Update it again: " . GermanShepherd::$num_of_dogs_created . "<br>";

Our GermanShepherd class is included and instantiated. In order to call the static members, we do not need to instantiate it, but once we do, we have a call within our constructor that updates the number of dogs created.

<?php
require_once("Dog.php");

class GermanShepherd extends Dog
{

    static int $num_of_dogs_created = 0;

    public static function update_num_of_dogs_created() {
        self::$num_of_dogs_created++;
    }

    public function __construct(string $eye_color,
                                string $dob,
                                string $fur_color)
    {
        $this->eye_color = $eye_color;
        $this->dob = $dob;
        $this->fur_color = $fur_color;

        self::update_num_of_dogs_created();
    }
  
    //...
}

To access the update_num_of_dogs_created() from our constructor, we have to use the self keyword since it is a static method. We can’t use $this. We have to use self followed by the scope-resolution operator, and finally by the static method name.

The static method increments the static property $num_of_dogs_created, again by appending self to the property name: self::$num_of_dogs_created++;

Let’s create one more non-static method: get_num_of_dogs_created_plus_two(). It’s just something random; don’t read too much into it :). All this method will do is, you guessed it, return the number of dogs created and add 2 to it.

<?php
require_once("Dog.php");

class GermanShepherd extends Dog
{

    static int $num_of_dogs_created = 0;

    //...

    public function get_num_of_dogs_created_plus_two() {
        return 2 + self::$num_of_dogs_created;
    }

    //...
}

This is just to illustrate that you can call a non-static method that returns a static value. Let’s call it and see what we get.

The output should look like the following:

Number of instances created: 1
Update it again: 2
Number of dogs create + 2: 4

Walking through the code:

• We’re calling a static property, $num_of_dogs_created, which returns 1.
• We’re then calling a static method, update_num_of_dogs_created(), which increments the static property to 2.
• The static property, $num_of_dogs_created is called again to display 2.
• Finally, we’re calling an instance method, get_num_of_dogs_created_plus_two() that returns the value of the static property $num_of_dogs_created incremented by 2.

That’s all there is when it comes to the self keyword.