PHP 登录系统/会话第一次不起作用

诊断问题有点困难，如果我的描述有点模糊，很抱歉。 我有一个非常标准的登录系统，并且在大多数情况下工作正常。 我今天注意到一个问题，虽然当我重新启动浏览器时......它不会让我登录 Admin1.php除非我先访问 admin2.php 并导航回 admin1.php。

之后一切正常。我可以注销并重新登录就好了。

这是我拥有的两个文件...

管理员1.php

<style>
#form_contain {
    text-align: center;
}
form {
    display: inline-block;
    margin: 100px 0 0 !important;
}
label {
    display: table-cell !important;
    vertical-align: middle;
}
input[type='text'], input[type="password"] {
    float: left;
    display: table-cell !important;
    margin: 0 0 10px;
    height: 25px;
    width: 260px !important;
}
</style>
<?php
session_start();
include "lib/head.php";
function protect($string)
{
 $string = mysql_real_escape_string($string);
 return $string;
} 
$user = $_SESSION['username']; 
if(!$_POST['submit'])
{
    if ($_SESSION['logged'] == true) {
        include_once("admin_main.php");
        }
    else {
?>
<div id="form_contain">
    <form method="post" action="<?=$_SERVER['PHP_SELF']; ?>">
        <label>Username:</label>
        <input type="text" name="username" maxlength="16">
        <br>
        <label>Password:</label>
        <input type="password" name="password" maxlength="16">
        <br>
        <br>
        <br>
        <input type="submit" name="submit" value="Login" class="btn">
    </form>
    <p id="alert"></p>  
</div>

<?php
    }
}
else
{
  $user = protect($_POST['username']);
  $pass = protect($_POST['password']);
if($user && $pass)
{
    $sql="SELECT id,username FROM catalog_users WHERE username='$user' AND password='$pass'";
    $query=mysql_query($sql) or die(mysql_error());
        if(mysql_num_rows($query) == 1)
        {
            $row = mysql_fetch_assoc($query); // mysql_fetch_assoc gets the value for each field in the row
            $_SESSION['id'] = $row['id']; //creates the first session var
            $_SESSION['username'] = $row['username']; // second session var
            $_SESSION['logged'] = true;
            echo '
                <script language="javascript">
                    window.location = "admin1.php";
                </script>";
            ';
        }
        else
       {
            echo '
                <script language="javascript">
                    window.location="admin_wrong.php";    
                </script>
            ';
        }   
}
else
{           
    echo '
        <script language="javascript">
            window.location="admin_wrong.php";    
        </script>
    ';
}
}
?>

管理员2.php

<?php
session_start();
include('lib/head.php');

if (isset($_SESSION['logged']) && $_SESSION['logged'] == true) {
    echo "
        This is a secured page, welcome <b>".$_SESSION['username']."</b>
        <br>
        <br>
        <a href='admin1.php'>Back</a>
        <a href='logout.php'>Logout</a>
        ";
    }
else {
    echo "<b>Sorry, you cannot access this page</b>";
}   

?>

更新：在我的代码顶部<?php session_start()之前的 CSS 导致了问题