我的代码出了什么问题? 9

here is my code:

Notice: Undefined index: id in C:\xampp\htdocs\3\header.php on line 36 Notice: Undefined index: id in C:\xampp\htdocs\3\header.php on line 47

    <tr>
        <td>
                <table align=center>
                <tr align=center>
                        <td><a href="index.php">Ask</a> | </td>
                        <td><a href="search.php">Questions</a> | </td>
                        <?php if ($_SESSION['id'] == ""): ?>
                        <td><a href="login.php">Login</a> | </td>
                        <td><a href="register.php">Register</a></td>
                        <?php else: ?>
                        <td><a href="expdir.php">Expert Directory</a> | </td>
                        <td><a href="logout.php">Logout</a> </td>
                        <?php endif; ?>
                </tr>
                </table>
                <table align=center>
                <tr align=center>
                    <?php if ($_SESSION['id'] != ""): ?>
                        <td><a href="cpanel.php">My Control Panel</a> | </td>
                        <td><a href="search.php?id=<?php echo $_SESSION['id']; ?>
">My Questions</a> | </td>
                        <?php if ($_SESSION['type'] == 'expert'): ?>
                                <td><a href="feedback.php">Feedback</a> | </td>
                        <?php endif; ?>
                        <td><a href="pm_inbox.php">Private Messenger</a> | </td>
                        <td><a href="reports.php?action=Accepted">Reports</a> | </td>
                        <td><a href="contact.php?action=Accepted">Contact</a> </td>
                    <?php endif; ?>
                </tr>
                </table>
        </td>
</tr>
<?php if ($_SESSION['id'] != ""): ?>

should be

<?php if (isset($_SESSION['id']) && $_SESSION['id'] != ""): ?>

You get the warning because $_SESSION['id'] it's undefined. How I would do that,

At the top of your page I Initialize a variable $sessionid

 $sessionid = (isset($_SESSION['id']) ? $_SESSION['id'] : NULL;

Then the check would be in this way

if(!empty($sessionid)){
 //do something
}