adding new key value is not working - php

Question

This is my json link which stored in variable named $onceBooking.
I want to count total booking days. If booking array is empty then add total_days = 0 but but if it is not empty then i am counting the unique booking_slots and adding them into main array. I am getting the correct count but when i am adding this into main it showing me this error.
$data = json_decode($onceBooking);
for ($i = 0; $i < count($data); $i++) {
if (count($data[$i]->bookings) == 0) {
$data[$i]->total_days = 0;
} else {
$bookings = $data[$i]->bookings;
for ($j = 0; $j < count($bookings); $j++) {
$booking_slots = $bookings[$j]->booking_slots;
$final_array = array();
$uniquekeys = array();
foreach ($booking_slots as $key => $data) {
if (!in_array($data->date, $uniquekeys)) {
$uniquekeys[] = $data->date;
$final_array[$key] = $data;
}
}
}
$data[$i]->total_days = count($final_array);
}
}
return $data;

Answer 1

Change your call to json_decode() to return an array by adding true as the 2nd argument.
// change this
$data = json_decode($onceBooking);
// to this
$data = json_decode($onceBooking, true);