OOP_PHP_InheritanceAbstractionEncapsulationPolymorphism

<?php

interface Introducable {
  function introduceYourSelf();
}

class Person implements Introducable {
  const PI = 3.14;
  private $name;
  private $family;
  private $age;
  private static $counter = 0;
  // public readonly $size = 0;

  function __construct($name, $family, $age) {
    $this->name = $name;
    $this->family = $family;
    $this->age = $age;
    self::$counter++;
  }

  function introduceYourSelf() {
    return "My name is $this->name. I am $this->age years old!";
  }

  public static function getCounter() {
    return self::$counter;
  }


}

class Student extends Person {
  private $fNumber;

  function __construct($name, $family, $age, $fNumber) {
    parent::__construct($name, $family, $age);
    $this->fNumber = $fNumber;
  }

  function introduceYourSelf() {
    return parent::introduceYourSelf() . "FNumber = $this->fNumber";
  }

}

$stoyan = new Person("Stoyan", "Cheresharov", 54);
$kircho = new Student("Kircho", "Ivanov", 24, "312312312");
?>

<h1>Hello world!</h1>
<pre>
<?php

echo  $stoyan->introduceYourSelf() . PHP_EOL; // My name is Stoyan. I am 54 years old!

echo  Person::getCounter() . PHP_EOL;
echo  $stoyan::getCounter() . PHP_EOL;
echo  $stoyan->getCounter() . PHP_EOL;
echo  Person::PI . PHP_EOL;
echo  $stoyan::PI . PHP_EOL;

echo $kircho->introduceYourSelf() . PHP_EOL; // My name is Kircho. I am 24 years old!FNumber = 312312312

// phpinfo();
?>
</pre>