会话变量未在服务器中设置，但在localhost中正常工作

Question

此代码在localhost中运行良好，但是当我将其上传到服务器时，它不会设置会话。

 <?php
    session_start();
    if (isset($_SESSION["manager"])) {
        header("location: index.php"); 
        exit();
    }
    ?>
    <?php
    // Parse the log in form if the user has filled it out and pressed "Log In"
    if (isset($_POST["username"]) && isset($_POST["password"])) {
        $manager = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["username"]); 
        $password = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["password"]);
        // Connect to the MySQL database  
         include "../storescripts/connect_to_mysql.php"; 
         $sql = mysql_query("SELECT id FROM admin WHERE username='$manager' AND password='$password' LIMIT 1"); // query the person
        // ------- MAKE SURE PERSON EXISTS IN DATABASE ---------
        $existCount = mysql_num_rows($sql); // count the row nums
        if ($existCount == 1) { // evaluate the count
             while($row = mysql_fetch_array($sql)){ 
                 $id = $row["id"];
             }
             $_SESSION["manager_id"] = $id;
             $_SESSION["manager"] = $manager;
             $_SESSION["manager_password"] = $password;
             header("location: index.php");
             exit();
        } else {
            echo 'That information is incorrect, try again <a href="index.php">Click Here</a>';
            exit();
        }
    }
    ?>
    <?php include_once("template_header.php");?>
    <div class="main">
            <div class="main_content">
                <div align="left" style="margin-left:24px;">
                    <h2>Please Log In To Manage the Store</h2>
                    <form id="form1" name="form1" method="post" action="admin_login.php">
                        <br>

                        <input name="username" type="text" placeholder="Username" id="username" size="40" />
                    <br /><br />

                        <input name="password" type="password" placeholder="Password" id="password" size="40" />
                    <br />
                    <br />

                        <input type="submit" name="button" id="button" value="Log In" />

                    </form>
                    <p>&nbsp; </p>
                    </div>
            </div>
    </div>
    <?php include_once("template_footer.php");?>