第三个ajax下拉列表返回错误的结果

I have 3 dropdown list (semester,section and subject) if I choose a semester it will populate section. Now if I choose a section it will populate subject but it does not read semester. it just display all the available subjects of the chosen section not considering what semester it is.

function showSEMESTER(semester) 
{
    var item = semester;
    var dataString = 'semester='+ semester;
    $.ajax
    ({
        type: "POST",
        url: "class-select.php",
        data: dataString,
        cache: false,
        success: function(html)
    {
        $("#class_id").html(html);
    } 
    });
}
function showSubject(section) 
    {
        var item = section;
        var dataString = 'code='+ item;
        $.ajax
        ({
            type: "POST",
            url: "subject-select.php",
            data: dataString,
            cache: false,
            success: function(html)
        {
            $("#subject_id").html(html);
        } 
        });
    }

here is the sql for subject select

    $query = "select * from subject where class_id = '$_POST[code]' and semester_id='$_POST[semester]' ";

this does not return any $_post[semester]

change

$query = "select * from subject where class_id = '$_POST[code]' and semester_id='$_POST[semester]' ";

to

$query = 'select * from subject where class_id = "'.$_POST['code'].'" and semester_id="'.$_POST['semester'].'"';

$_POST is an array with string indexes. You lack quotes "" or '' for the $_POST indexes.