问如何创建第二个下拉菜单以减小单个下拉菜单列表大小

EN

<?php
    $arr = ["city_code1" => "city_name1", 
            "city_code2" => "city_name2", 
            ---
            "city_codei" => "city_namei"] 

    $city = isset($_POST['city']) ? $_POST['city'] : array_keys($arr)[0];
?>

<form name="f" id="a" method="post" action="">
    <select id="city" name="city" onchange="this.form.submit()" >                      
    <?php
        foreach ($arr as $k => $v) {
            echo "<option value='$k'" . ($k == $city ? " selected" : "") . ">$v</option>\n";
        }
    ?>
    </select>
</form>

<?php
    //create url
    $city_name = $arr[$city];
    $fIOURL = "http://www.exemple.com/{$city_name}";

    // check cache
    $city_cache = "cache/{$city_name}.txt";
    $cache_exists = file_exists($city_cache);
    if (!$cache_exists || time() - filemtime($city_cache) > 60 * 60 * 3) { 
        // cache doesn't exist, or is no longer valid
        $rawData = file_get_contents($fIOURL);
        if ($rawData != "") {
            // if we successfully fetched data, recreate the cache
            $cache_exists = file_put_contents($city_cache, $rawData);
        }
    }

    if ($cache_exists) {
        // fetch the data (either cached or freshly loaded) from the cache file
        $rawData = file_get_contents($city_cache);
        $forecastLoadedTime = filemtime($city_cache);
    }
    else {
        // some sort of error message here
        $rawData = "Error no forecast available for $city_name!";
    }

    $decoded = json_decode($rawData, true);