Laravel - Compare arrays and update if data was different

Here's the flow: I select up to 5 students in a listing and click the "Generate Training" button.
Then I record in the session an array of the selected students.
In the next screen I retrieve this array and render a list with a checkbox called "Attended" where you can select whether the student went to training or not. By clicking on the "Record Training" button I must update the students who attend the training. So far so good.
The problem is, I can't update the original array that contains all the students. For example, if 5 students were selected but only 3 attended the training, I should update the training of those students in the database and update the original array, keeping the data that was not changed along with the data that was changed.
The code below shows how I am trying to upgrade the main array (with all students selected). The code is commented, including the errors that appear:

// dd($ar_student); //ARRAY WITH STUDENTS WHO ENJOYED TRAINING
    // dd($all_students);//ARRAY WITH ALL SELECTED STUDENTS (MAY BE FROM 1 TO 5)
    foreach($all_students as $item1){//I GO ARRAY TO ALL STUDENTS
        foreach($ar_student as $item2){//I GO TO ARRAY THE STUDENTS ATTENDING
            // dd($item1->student_id);
            if($item1->student_id == $item2->student_id){//IF IDS ARE EQUAL
                // dd($item2);
                $all_students->fase_id = $item2->fase_id;//UPDATE ARRAY WITH ALL STUDENTS (REALLY I NEED TO UPDATE STUDENT PHASE ONLY). ERROR AT THIS LINE: Attempt to assign property of non-object
            }
        }
    }
    $students = $all_students;
    //dd($all_students);

via Chebli Mohamed