在smarty模板中访问变量值有什么问题?

我从PHP文件中分配一个值给smarty模板。我的PHP文件代码片段如下:

<?php  
  require_once("includes/public-application-header.php");
  ob_start();
  prepare_request();
  $request = empty( $_GET ) ? $_POST : $_GET ;
  $op = $request['op'];
  $objTeacherDetails = new TeacherDetails();
  $teacher_id = $_GET['teacher_id'];
  $teacher_classes = $objTeacherDetails->GetAllClassesByTeacherId($teacher_id);
    $smarty->assign('teacher_classes', $teacher_classes);
  $file_to_show = "teacher-details.tpl";
switch( $op ) {
    case "get_assigned_subject_list":
        $objClassSubjects = new ClassSubjects();
        $objSubjects = new Subjects();
        $class_id  = $request['class_id'];
        $all_subjects = $objSubjects->GetAllSubjects();
        $subject_details = $objClassSubjects-> GetClassSubjectDetailsById($class_id);
        $smarty->assign('all_subjects', $all_subjects);
        $smarty->assign('subject_details', $subject_details);
        $smarty->assign('teacher_id', $teacher_id); 
        $smarty->display("assigned-subject-list.tpl");
        die();
        break;
?>

我的问题是我无法访问smarty模板中的$teacher_id的值。我的smarty模板代码如下:

<input type="text" value="{$teacher_id}" name="tid">

但是我的文本字段是空白的。有人能帮我解决这个小问题吗?