Переберите циклом массив $arr и с его помощью найдите сумму зарплат работников

<?php
class Employee
{
    public $name;
    public $salary;

    public function __construct($name,$salary)
    {
        $this->name = $name;
        $this->salary = $salary;
    }
}   


class Student
{
    public $name;
    public $scholarship;

    public function  __construct($name, $scholarship)
    {
        $this->name = $name;
        $this->scholarship = $scholarship;
    }
}



    $employee1 = new Employee('Sefer', 40.000);
    $employee2 = new Employee('Edem', 30.000);
    $employee3 = new Employee('Asia', 20.000);

    $student1 = new Student('Roma', 100);
    $student2 = new Student('Liza', 150);
    $student3 = new Student('Sasha', 200);

    $arr = [$employee1, $employee2, $student1, $student3, $employee3, $student2];

    $sum = 0;
    foreach ($arr as $elem) {
    if ($elem instanceof Employee) {
    for ($i = 0; $i < count($elem->salary); $i++) {
        $sum += $elem[$i];
        echo $sum;
            }
        }

    }