phpmysql+セッションの問題

index.php内 'session_start（）;'

の後に条件を最上位に置く必要があります

if($_SESSION['username'])
{
    header("Location: home.php");
    exit();
}

whileループでは、header("Location: home.php");である必要があります。 header("Location: index.php");の代わりに

home.php内 phpタグを開いた後に一番上に置くべきページ

ob_start();
session_start();

それがうまくいくことを願っています。

++++++++++++++++++++++++++++++++++++++++++

このコードを使用するindex.php

<?php
require_once('connect.php');
ob_start();
session_start();
//checked wether the user is loged in  or not 

$user = $_SESSION['username'];

if($_SESSION['username'])
{
    $user = $_SESSION['username'];
    header("Location: home.php");
    exit();
}

// login script
if(isset($_POST['username'])&& isset($_POST['password']))
{
    $user_login = preg_replace('#[^A-Za-z0-9]#i', '', $_POST['username']);
    $user_password = preg_replace('#[^A-Za-z0-9]#i', '', $_POST['password']);
    $md5password = md5($user_password);
    $sql = mysql_query("SELECT id FROM members WHERE username = '".$user_login."' AND password = '".$user_password."'") or die ("could not select from database");

    $userCount = mysql_num_rows($sql);
    if($userCount ==1)
    {
        while($row = mysql_fetch_array($sql))
        {

            $id = $row['id'];
        }

        $_SESSION['id'] = $id;
        $_SESSION['username'] = $user_login;
        $_SESSION['password'] = $user_password;
        header("Location: home.php");
        exit();
    }
    else
    {
         echo "that info is incorrect";
         exit();
    }
}


?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>

<body>
<form action="login.php" method="post">

<input name="username" type="text" value="username" size="32" />
<input name="pass" type="password" value="password" size="32" />
<input name="login" type="submit" value="login" />

</form>

</body>
</html>
<?php  ob_end_flush(); ?>

home.php

<?php
ob_start();
session_start();

//home.php
$user = $_SESSION['username'];
if(!isset($_SESSION['username']))
{
    header("Location: index.php");
    exit();
}
else
{

    echo "hi $user you are loged in //Welcome to our website <a href=\"logout.php\">Logout</a>";


}
?>

logout.php 正しい