Php 选择和显示数据取决于下拉菜单的值

我想根据下拉菜单进行选择，这是我的代码

视图：- -选课-

控制器：-

public function select_student($class_id){ <br />$class_id=$this->uri->segment(3);<br /> $this->load->model('select_model');<br />    header('Content-Type: application/x-json; charset=utf-8');<br />  echo(json_encode($this->select_model->student_subject($class_id)));<br />

}

这是我的模型

查看页面

<select name="class" id="class">
    <option>Select class</option>
    <?php foreach($classes as $class){?>
    <option value="<?php echo $class['class_id'] ?>">
        <?php echo $class['class_name'] ?>
    </option>
    <?php } ?>
</select>

---

没有人，格式化你的代码以便下一个人可以阅读

<select name="class" id="class">
    <option>Select class</option>
    <?php foreach($classes as $class){?>
    <option value="<?php echo $class['class_id'] ?>">
        <?php echo $class['class_name'] ?>
    </option>
    <?php } ?>
</select>