Change Login button to logout

Question

First of all I want to warn you that I am a beginner in PHP and I appreciate help because I need to deliver a work and I am in Deadline

I have a login button and I want you to switch to logout when the session is started but the condition works for multiple pages!

This is the code of my login.php page to which the login button sends the user when it is clicked!:

        <div class="inner_container">
            <label><b>Username</b></label>
            <input type="text" placeholder="Enter Username" name="username" required>
            <label><b>Password</b></label>
            <input type="password" placeholder="Enter Password" name="password" required>
            <button class="login_button" name="login" type="submit">Login</button>
                            <a href="registo.php"><button type="button" class="register_btn">Criar conta</button></a>
        </div>
    </form>

    <?php
        if(isset($_POST['login']))
        {
            @$username=$_POST['username'];
            @$password=$_POST['password'];
            $query = "select * from user where username='$username' and password='$password' ";

            $query_run = mysqli_query($con,$query);

            if($query_run)
            {
                if(mysqli_num_rows($query_run)>0)
                {
                $row = mysqli_fetch_array($query_run,MYSQLI_ASSOC);

                $_SESSION['username'] = $username;
                $_SESSION['password'] = $password;
                header( "Location: index.php");
                }
                else
                {
                    echo '<script type="text/javascript">alert("Nome de utilizador ou palavra passe inválida")</script>';
                }
            }
            else
            {
                echo '<script type="text/javascript">alert("Database Error")</script>';
            }
        }
        else
        {
        }
    ?>

</div>

Answer 1

Put this on the pages that would like to appear the buttons, exchange the link for a button.

if(isset($_SESSION['username']))
{
   echo '<a href="index.php?logout=1">LOGOUT</a>';
}
else
{
   echo '<a href="login.php">LOGIN</a>';
}

On the index.php page put before any code

$logout = isset($_GET['logout']) ? $_GET['logout'] : "";

if ($logout == 1)
{
unset($_SESSION['username']);
}

On restricted pages, you can enter this code below to prevent access if the session variable is null.

if(!isset($_SESSION['username']))
{
    echo '
        <div class="erro">
        Não tem permissão para ver o conteúdo desta página.<br>
        <a href="index.php">Voltar</a>
        </div>';
        exit;
}