Objects in PHP 6/10

I need help… I can’t find my mistake.. :(

class Person { public $isAlive = true; public $firstname; public $lastname; public $age; public function greet() { return “Hello, my name is “.$this->firstname.” “.$this->lastname.”. Nice to meet you! :-)”; }

  public  function __construct($firstname, $lastname, $age) {
    $this->firstname = $firstname;
    $this->lastname = $lastname;
    $this->age = $age;
  }
}

  $teacher = new Person("boring", "12345", 12345);
  $student = new Person("Mike", "Ruderisch", 16);
  echo $teacher->isAlive;
  echo $student->age;
  echo $student->greet();
  echo $teacher->greet();

?>