Choices Callback is not saving data

i created a dropdown(parent) and the second one with parent choices callback
i use this function

function getcity() {
$choice = $_POST[‘parent_option’];
switch($choice) {
case “Andaman & Nicobar”:
$city = [
“Alipur” => “Alipur”,
“Andaman Island” => “Andaman Island”,
“Anderson Island” => “Anderson Island”,
“Arainj-Laka-Punga” => “Arainj-Laka-Punga”,
“Austinabad” => “Austinabad”,
“Bamboo Flat” => “Bamboo Flat”];
break;
default: $city = [——];
}
return $city;
}

this works exactly what i want in forntend but when i am submitting any information with the second choices callback filed it’s not saving the information(first parent dropdown is saved there is no problem with that).

help please!!