cookie not working in php login

Question

i have written a php login script which uses both session and cookies. i know that sessions expire after closing the browser,but cookies exist till their expiration time is reached. in my script i created a COOKIE to store "username" of the logged in user. After logging in , if i close the browser and re-open it..i must still remain logged in because of the cookie.And when i click log out i must be logged out. But my script is not working properly because,after clicking the "logout" button link my script is not logging me out.

Here's the php code that i wrote:

    <?php
session_start();
require_once('login/connectvars.php');
    if(!isset($_SESSION['username']) && isset($_COOKIE['username']))
{
    $_SESSION['username'] = $_COOKIE['username'];
}

if(isset($_POST['submit']))
        {
          if(!empty($_POST['username']) && !empty($_POST['password']))
          {
            $dbc = mysqli_connect(DB_HOST,DB_USER,DB_PASSWORD,DB_NAME);
            $username = mysqli_real_escape_string($dbc,trim($_POST['username']));
            $password = mysqli_real_escape_string($dbc,trim($_POST['password']));

            $query = "SELECT user_id,username FROM users_dns WHERE username = '$username' AND password = SHA('$password')";
            $data = mysqli_query($dbc,$query) or die(mysqli_error($dbc));
            if(mysqli_num_rows($data) == 1)
            {
                $row = mysqli_fetch_array($data);
                $_SESSION['user_id'] = $row['user_id'];
                $_SESSION['username'] = $row['username'];
                setcookie('username',$row['username'],time() + (60 * 60));
                echo 'success';
                mysqli_close($dbc);
            }    
            else
            {
                echo 'enter correct username and password';
            }
         }
        else
        {
            echo 'no empty fields please..';
        }
      }

?>

Here's what i used in the html code:

<?php
       if(isset($_SESSION['username']))
       {
         echo 'logged in as'.$_SESSION['username'];
         echo '<a href="login/logout.php">log out ('.$_SESSION['username'].')</a><br />';
         echo $_SESSION['username'];
       }
       else
       {
       ?>
       <div id="login_form">

        <!--<span class="error"><?php echo $error_msg; ?></span>-->
         <form id="login"  method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
            <table>
                  <tr>
                     <th>Username:</th>
                     <th>Password:</th>
                  </tr>
                  <tr>
                     <td><input type="text" name="username" /></td>
                     <td><input type="password" name="password" /></td>
                   </tr>
                   <tr>
                      <td><input type="submit" name="submit" value="Log In" class="submit" /></td>
                      <td><a href="login/signup.php"><input type="button" class="submit" value="Sign Up" /></a></td>
                   </tr>
            </table>
         </form>
       </div>
       <?php
       }
       ?>

Here is my log out script

    <?php
session_start();

if(isset($_SESSION['username']))
{
    $_SESSION = array();

if(isset($_COOKIE['session_name()']))
{
    setcookie('username','',time() - 3600);
}
session_destroy();
}
setcookie('user_id','',time() - 3600);
setcookie('username','',time() - 3600);

echo 'redirecting you...please wait..';
header("Refresh: 3;url=http://localhost/");
?>

Answer 1

if(isset($_COOKIE['session_name()']))

this should be

if(isset($_COOKIE['username']))

because you have created username cookie and you are checking for session_name() cookie.