提示undefined 怎么处理

<?php

if(isset($_REQUEST['authcode'])){

session_start();

if(strtolower($_REQUEST['authcode']==$_SESSION['authcode'])){

echo '<font color = "#0000CC">输入正确</font>';

}else{

echo '<font color = "#CC0000"><b>输入错误</b></font>';

}

exit();

}

?>

提示是Notice: Undefined index: authcode in E:\wamp\www\yanzheng\form.php on line 4