Adding values of an array N times to reach a target - php

Question

I have a target number (maximum capacity of more than one class room)
An array of department IDs .
$depts = array(1,2,3);
A function that returns the total number of students taking a particular course from a Department.
function noStudentsTakingCourse ($dept_id){
//Some expressions..
return $student_count;
}
An array of capacity of the available rooms, ie, the capacity of class rooms that are vacant
$available_room_capacities = array(50, 75, 40, 36, 100, 250);
What I want is:
From the department IDs, I want each available class room to be assigned a department in which the number of students taking that in the department is greater or equal to the room capacity.
I don't know if you get me, but am fairly new to programming and I've spent days trying to get this without any success.
$total_no_of_students_taking_course = 223;
$dept_ids_taking_course = array(1, 2, 3);
$available_room_capacities = array(50, 70, 35, 59, 95, 100);
function noOfStudentsTakingCourse($dept_id)
{
$r = 0;
if ($dept_id == 1) {
$r = 100;
} elseif ($dept_id == 2) {
$r = 56;
} elseif ($dept_id == 3) {
$r == 67;
}
return $r;
}
Hers's what i've been able to come up with. all it does is randomly sum values from the class_room array until they're greater than the total number of students taking the course. But, most times it sums more than 5 values when the department here is only 3. i want the case where by:
- Department 1 (with capacity of 100) would be allocated a room greater of equal to 100,
- Department 2 (with capacity of 56) would be allocated a room greater or equal to 56 and so on.
That means the final array would be of lenght 3 and its sum would not be less than 223 (according to the code above)
THANKS IN ADVANCE