how to prevent array_merge to renumber numeric keys

use '+' operator instead of array_merge():

function array_to_list($arr_data, $str_key, $str_value) {
$arr_list = array();
if (is_array($arr_data)) {
    foreach($arr_data as $arr_value) {
        if (isset($arr_value[$str_key]) && isset($arr_value[$str_value])) {
            $arr_list = $arr_list + array($arr_value[$str_key] => $arr_value[$str_value]);
        }
    }
}
return $arr_list;

}

Why don't you just try a much simpler approach:

function array_to_list($arr_data, $str_key, $str_value) {
    $arr_list = array();
    if (is_array($arr_data)) {
        foreach($arr_data as $arr_value) {
            if (isset($arr_value[$str_key]) && isset($arr_value[$str_value])) {

                // This is the changed line:
                $arr_list[ $arr_value[$str_key] ] = $arr_value[$str_value];

            }
        }
    }
    return $arr_list;
}

This just sets the values on the output array directly. There's no reason to make a new array and merge it each time. It also should be a bit faster.