问如何在codeigniter的控制器上传递jquery变量？

EN

<select name="cities" id="cities" onChange="change_city();">    
    <option value="Basel">Basel</option>
    <option value="Bern">Bern</option>
    <option value="Fribourg">Fribourg</option>
    <option value="Geneva">Geneva</option>
    <option value="Lugano">Lugano</option>
    <option value="Zurich">Zurich</option>
</select>

function change_city(){
    var home_city =$("#cities option:selected").val();
    var params = 'city='+home_city;
    $.ajax({
       type: "POST",
       url: lang+"/webshop/change_city",
       data: params,
       success: function( r ) {
         $('#ajax_msg').html( r );
         location.reload( true );
       }
    });
}

function change_city(){
        $city = $_POST['city'];
        $expire = 9999999999;

        setcookie("home_city", $city, $expire);
    }