Class usort

<?php
class Student{
	public $FirstName;
	public $LastName;
	public $UID;
	public $Achivment;
	public $AverageScore;
	
	public function __construct(string $FirstName, string $LastName , array $Achivment){
		$this->FirstName = $FirstName;
		$this->LastName = $LastName;
		$this->Achivment = $Achivment;
		$this->AverageScore = $this->getAverageScore();
	}
	
	public function getAverageScore(){
		return array_sum($this->Achivment) /count($this->Achivment);
	}
	
	public static function SortBy($obj_ar, $sortBy,$order) {
		usort($obj_ar, function($object1, $object2) use ($sortBy,$order) {
			switch($order){
				case "DESK":
					return $object2->$sortBy <=> $object1->$sortBy;
				case "ASK":
					return $object1->$sortBy <=> $object2->$sortBy;
			} 
		});
		return $obj_ar;
	}
}

$students = [
	
	
	new Student("Sidr","Sidorov",['Математика'=>3,'Русский'=>5,'Информатика'=>4]),
	new Student("Petr","Petrov",['Математика'=>3,'Русский'=>4,'Информатика'=>3]),
	new Student("Ivan","Ivanov",['Математика'=>5,'Русский'=>5,'Информатика'=>5]),
];
//$sorted_students = Student::SortBy($students, "AverageScore",'ASK');
$sorted_students = Student::SortBy($students, "AverageScore",'ASK');
?>
<ul>
	<?php 
		foreach ($sorted_students as $student ){
		?>
			<li><?php echo 'Средний балл студента-'. $student->LastName . ' равен '.$student->getAverageScore(); ?></li>
		<?php
		}
	?>
</ul>

Results:

Средний балл студента-Petrov равен 3.3333333333333
Средний балл студента-Sidorov равен 4
Средний балл студента-Ivanov равен 5