Optional setter - php

Question

How can I set a specific setter inside of a child class to be optional
I have a web page build with PHP that prints a recursive objects array, I have a parent class called Person and a child class called Teachers.
Now the teachers can also teach another teacher, I have divided my teachers with the following types "Master, Senior, Junior", and as a matter of fact I already solve that problem, I can see an array of objects within an array of objects, something like this:
Array
(
[0] => Teacher Object
(
[subject:Teacher:private] => Matematicas
[teacherType:Teacher:private] => Master
[teachers:Teacher:private] => Array
(
[0] => Teacher Object
(
[subject:Teacher:private] => Matematicas 2
[teacherType:Teacher:private] => Senior
[teachers:Teacher:private] => Array
(
[0] => Teacher Object
(
[subject:Teacher:private] => Ciancias 1
[teacherType:Teacher:private] => Junior
[teachers:Teacher:private] => Array
(
)
[id:Person:private] => 7
[name:Person:private] => Pedro
[nickname:Person:private] => PedroBoy
)
)
I imagine you get the idea, my problem is that the junior teacher does not receive any teachers array, because he is a jnuior teacher and he can't teach teachers yet. But if I write nothing on it, it will launch an error saying that the teachers array of junior teachers is undifined and it will assign an empty teachers array object to it.
Is there any way to nake that specific setter optional?.
Here is my PHP Code:
class Teacher extends Person{
private $subject = '';
private $teacherType = '';
private $teachers = array();
public function __construct($data) {
parent::__construct($data);
$this->setSubject($data["subject"]);
//OPTIONAL
$this->setTeachers($data["teachers"]);
$this->setTeacherType($data["type"]);
}
?>

Answer 1

Maybe a simple if statement?
class Teacher extends Person
{
private $subject = '';
private $teacherType = '';
private $teachers = array();
public function __construct($data)
{
parent::__construct($data);
$this->setSubject($data["subject"]);
if (isset($data['teachers']))
$this->setTeachers($data["teachers"]);
$this->setTeacherType($data["type"]);
}