会话数据在页面之间丢失

如果我在主页上登录，我会遇到问题，然后我会转到另一个页面，再也看不到存储的会话，但当我单击登录时，它会显示我已经登录。但如果我转到其他目录，如/admin/，会话数据仍在。有什么解决方案吗？

login.php

<?php 
session_start(); 
require_once('connections/Main.php');
if($_SESSION['username']) {
    echo '<div class="error_message">Attention! '.$_SESSION['username'].'. You  are already logged in.</div>';
    echo "<br />";
    echo "Go <a target='top' href='index.php'>back</a> to the page you were viewing before this.</li>";
    exit();
}
// Has an error message been passed to login.php?
$error = $_GET['e'];
if($error == 1) {
    $error = '<div class="error_message">Attention! You must be logged in to view this page.</div>';
}
// Only process if the login form has been submitted.
if(isset($_POST['login'])) {
    $username = $_POST['username']; 
    $password = $_POST['password']; 
        if (empty($username) || empty($password)) { 
        $error = '<div class="error_message">Attention! Please enter your Username and Password.</div>';
    } else { 
     mysql_select_db($database_Main);
    // Add slashes to the username and md5() the password 
    $user = mysql_real_escape_string(addslashes($_POST['username'])); 
    $pass = mysql_real_escape_string(md5($_POST['password'])); 

    $sql = "SELECT * FROM  users WHERE username = '$user' AND password= '$pass' OR email = '$user' AND password= '$pass' "; 
    $result = mysql_query($sql);
    // Check that at least one row was returned 
    $rowCheck = mysql_num_rows($result); 
    if($rowCheck > 0) { 
    while($row = mysql_fetch_array($result)) { 
      // Start the session and register a variable 
      if(isset($_POST['remmemberme'])){
          setcookie(kitbusername, $user,  time()+(3600*24*30));
      }
      $_SESSION['username'] = $user;
      //session_register('username'); 
      echo '<script>parent.changeURL("index.php"); </script>';
      } 
      } else { 
      // If nothing is returned by the query, unsuccessful login code goes here... 
      $error = '<div class="error_message">Attention! Incorrect username or password.</div>'; 
      } 
    }
}
echo $error; ?>

每页上的header.php

<div id="loginoutbox">
<?php session_start(); 
  if($_SESSION["username"]) { ?>
<div>
  Hello <?php echo $_SESSION['username']; ?>! | </div>
<a id="register" href="http://www.kdogisthebest.webege.com/admin/" class="button">Dashboard</a> |
<a id="signin" href="#fallr-confirmlike" class="button">Logout</a>      
<?php }else{ ?>
<div style="display: inline-block; font-size: 14px; padding-left: 20px;">Hello Guest!</div>
<a id="register" href="#fallr-register" class="button">Register</a> |
<a id="signin" href="#fallr-signin" class="button">Login</a>
<?php } ?>
</div>

演示--->http://www.kdogisthebest.webege.com/index.php

用户名=脱模剂

密码=脱模器