php select表单提交未定义,该怎么处理

<form method="post" action="lib\ChkLogin.php"><br />    	..............................<br />            <select name="role"><br />            	<option value="student">student</option><br />                <option value="teacher">teacher</option><br />            </select><br />            </td><br />            </tr><br />            <tr><br />            <td><input type="submit" value="ok" /></td><br />            <td><input type="reset" value="no" /></td><br />            </tr><br />        <br />        </table><br />        </form>

<?php<br />$username = $_POST['username'];<br />$passwd = $_POST['passwd'];<br />$role = $_POST['role'];<br />if($username==""||$passwd==""){<br />	echo "<script>";<br />	echo "alert(\"用户名或密码不能为空！\");";<br />	echo "location.href=\"../index.php\";";<br />	echo "</script>";<br />	}<br />        ......................<br />