PHP dependent dropdown based on file columns

Hi, i have a dependent dropdown.

When you select county, you get the constituency and when you select the constituency you get the ward.

I am able to get the constituency from the county but i get an error Notice: Undefined index: Please choose a constituency in C:\xampp\htdocs\ramani_dsl\lidepot\dropdown.php on line 44 {"county":["MOMBASA","KWALE","KILIFI","TANA RIVER"]} when trying to get the ward.

My code looks like this :

<?php

    // read the CSV file in the $makes_models_years array

    $makes_models_years = array();
    //$uploads_folder = wp_upload_dir()['basedir'];
    chdir(__DIR__);
    $file = fopen("./resources/polling_data.csv","r");

    $firstline = true;
    while (($line = fgetcsv($file)) !== FALSE) {
    if ($firstline) {
        $firstline = false;
        continue;
    }
    $makes_models_years[$line[0]][$line[1]][] = $line[2];

    }
    fclose($file);

    // setup the initial array that will be returned to the the client side script as a JSON object.

    $return_array = array(
        'county' => array_keys($makes_models_years),
        'constituency' => array(),
        'ward' => array(),
        'current_county' => false,
        'current_constituency' => false
    );
   // print_r($return_array);
    // collect the posted values from the submitted form

    $make = key_exists('county', $_POST) ? $_POST['county'] : false;
    $model = key_exists('constituency', $_POST) ? $_POST['constituency'] : false;
    $year = key_exists('ward', $_POST) ? $_POST['ward'] : false;

    // populate the $return_array with the necessary values

    if ($make) {
    $return_array['current_county'] = $make;
    $return_array['constituency'] = array_keys($makes_models_years[$make]);
    if ($model) {
    $return_array['current_constituency'] = $model;
    $return_array['ward'] = $makes_models_years[$make][$model];
    if ($year) {
            $return_array['current_ward'] = $year;
        }
        }
    }

    // encode the $return_array as a JSON object and echo it
    echo json_encode($return_array);
  
?>

My csv looks like https://onedrive.live.com/edit.aspx?resid=53D2116F2F9AA49!225144&ithint=file%2cxlsx&authkey=!AOdTqj8O4Py0PQ0

Any pointers on how to get the ward from the ward will be appreciated.