php change sign in to sign out after user signs in

Question

signin.php:

Answer 1

In place of

<li><a href="signin.php">Sign in</a></li>

you need

<li>
    <?php
        if(isset($_SESSION['username'])) { ?>
            <a href="signout.php">Sign out</a>
    <?php 
        } else { ?>
            <a href="signin.php">Sign in</a>
    <?php
        } ?>
</li>

---

BUT

You have massive security issues there. You're storing passwords in plain text in your database, by the looks of it. You need to fix this. Now.