Access variable from other function LARAVEL - laravel

Question

I'm trying to use the returned result from a function into another function. Here is my first function:
public function availableRooms(){
if(isset($_POST['selectedDate'])){
$checkInDateReceived = str_replace('/', '-', $_POST['selectedDate']);
$checkInDate = date('Y-m-d', strtotime($checkInDateReceived));
$availableRooms = DB::table('rooms')
->leftJoin('reservations', function ($secondJoin) use($checkInDate) {
$secondJoin->where('reservations.checkout_date', '<', $checkInDate);
})
->leftJoin('reservation_room_rel', function ($firstJoin) {
$firstJoin
->on('reservation_room_rel.room_id', '=', 'rooms.id')
->on('reservation_room_rel.reservation_id', '=', 'reservations.id');
})
->where([
['rooms.status', '=', 1]
])
->whereNull('reservation_room_rel.room_id')
->get();
return $availableRooms;
}
}
And here is my second function:
public function returnAddReservation(){
$availableRoomsRet = $this->availableRooms();
$mealSupplement = new Meal;
$accommodationSupplement = new AccommodationSupplement;
$accommodationSupplements = $accommodationSupplement::where('status', 1)
->orderBy('name', 'asc')
->get();
$mealSupplements = $mealSupplement::where('status', 1)
->orderBy('name', 'asc')
->get();
return view('addReservation', [
'availableRooms' => $availableRoomsRet,
'accommodationSupplements' => $accommodationSupplements,
'mealSupplements' => $mealSupplements
]);
}
First function returns correct result if it runs independently, but, when I dd($availableRoomsRet)it returns null.
Any idea how to access properly $availableRooms from public function availableRooms() into public function returnAddReservation() ?
Thanks!