<?php

$b->router_add("/set-city/%", function () { city_save(); }, array("get"));

// Сохранить город
function city_save()
{
    global $b;
    $link = $b->router_get_segment(1);
    $b->session_set("city", $link);
    echo json_encode(array("ok" => true));
    exit();
}

// Получить текущий город
function get_city()
{
    global $b;
    $session_city = $b->session_get("city");
    $cities = $b->setting_get_value("cities");
    if($session_city === false) return $cities[0]["label"];
    foreach ($cities as $key => $city)
        if($city["link"] == $session_city) return $city["label"];
    return $cities[0]["label"];
}