Descriping the factory method design pattern

factorymethod.php

<?php

// Explaining the Factory Method Pattern
abstract class Student
{
    public $name;
    public $level;

    // The factory method doesn't know which type of certificates to issue
    // instead subclasses can define that type
    abstract public function issueCertificate();
}

abstract class Certificate
{
    public $score;
    abstract public function calculateResults();
}

class ElementaryStudent extends Student
{
    public function issueCertificate()
    {
        return new ElementaryCertificate();
    }
}

class PreparatoryStudent extends Student
{
    public function issueCertificate()
    {
        return new PreparatoryCertificate();
    }
}

class ElementaryCertificate extends Certificate
{
    public function calculateResults()
    {

    }
}

class PreparatoryCertificate extends Certificate
{
    public function calculateResults()
    {

    }
}

$ahmed = new ElementaryStudent();
$ahmed->name = 'Ahmed Mohammed Ibrahim';
$ahmed->age = 5;
$ahmedCertificate = $ahmed->issueCertificate();
$ahmedCertificate->score = 100;
var_dump($ahmedCertificate);

$mohammed = new PreparatoryStudent();
$mohammed->name = 'Mohammed Mohammed Ibrahim';
$mohammed->age = 12;
$mohammedCertificate = $mohammed->issueCertificate();
$mohammedCertificate->score = 170;
var_dump($mohammedCertificate);