A simple gist to show PHP Abstract class and Late Static Binding and Magic method.

php-abstract-class-late-static-binding.php

<?php
// Problem
/**
 * Review the code:
 * 1) Find errors
 * 2) What would be improved from the design point of view? 
 *    How to get rid of implementing not used methods in the child classes?
 * 3) Implement a counter for the created instances of any class from the hierarchy.
 * 4) Implement getCategory method so it returns the value of the constant 
 *    defined in the child class.
 */
class ParentEntity
{    
		abstract public function getName(): string;
    
    abstract public function getTitle(): string;
    
    abstract public function getDescription(): string;
    
    protected function getCategory(): string
    {
    	// TBD
    }
}

class FirstChildEntity extends ParentEntity
{
	public const ENTITY_CATEGORY = "first";

	public function getName(): string
    {
    	return "Child 1";
    }
    
    public function getTitle(): string
    {
    	return "Title 1";
    }
    
    public function getDescription(): string
    {
    	throw new NotImplementedException("getDescription function is not be implemented under the first child");
    }
}

class SecondChildEntity extends ParentEntity
{
	public const ENTITY_CATEGORY = "second";
    
	public function getName(): string
    {
    	throw new NotImplementedException("getName function is not be implemented under the first child");
    }
    
    public function getTitle(): string
    {
    	throw new NotImplementedException("getTitle function is not be implemented under the first child");
    }
    
    public function getDescription(): string
    {
    	return "Description 2";
    }
}

class ThirdChildEntity extends ParentEntity
{
    public const ENTITY_CATEGORY = "third";

	public function getName(): string
    {
    		return "Name 3";
    }
    
    public function getTitle(): string
    {
    		return "Title 3";
    }
    
    public function getDescription(): string
    {
    		return "Description 3";
    }
}

// Solution
abstract class ParentEntity
{
    static $count = 0;

    public function __construct() {
        static::$count++;
    }
	abstract public function getName(): string;

    abstract public function getTitle(): string;

    abstract public function getDescription(): string;

    protected function getCategory(): string
    {
    	return static::ENTITY_CATEGORY;
    }
}

abstract class FirstChildEntity extends ParentEntity
{
	public const ENTITY_CATEGORY = "first";
    static $count = 0;

	public function getName(): string
    {
    	return "Child 1";
    }

    public function getTitle(): string
    {
    	return "Title 1";
    }

    abstract public function getDescription(): string;

    public function getCategory(): string
    {
    	return parent::getCategory();
    }
}

class SecondChildEntity extends FirstChildEntity
{
	public const ENTITY_CATEGORY = "second";
    static $count = 0;



    public function getDescription(): string
    {
    	return "Description 2";
    }

    public function getCategory(): string
    {
    	return parent::getCategory();
    }

    public function getCounter() {
    	echo static::$count;
    }
}

class ThirdChildEntity extends SecondChildEntity
{
    public const ENTITY_CATEGORY = "third";
    static $count = 0;

	public function getName(): string
    {
    		return "Name 3";
    }

    public function getTitle(): string
    {
    		return "Title 3";
    }

    public function getDescription(): string
    {
    		return "Description 3";
    }

    public function getCategory(): string
    {
    	return parent::getCategory();
    }

    public function getCounter() {
    	echo static::$count;
    }

    public function __clone() {
    	static::$count++;
    }
}

$third = new ThirdChildEntity();
echo $third->getCategory() . PHP_EOL;

$second = new SecondChildEntity();
echo $second->getCategory()  . PHP_EOL;

// $first = new FirstChildEntity();
// echo $first->getCategory();
$obj1 = new ThirdChildEntity();
$obj2 = new ThirdChildEntity();
$obj2->getCounter()  . PHP_EOL;
$obj3 = clone $obj2;
$second->getCounter()  . PHP_EOL;
$obj3->getCounter()  . PHP_EOL;