PHP不保存用户会话

I’m kinda new to PHP and having a little trouble with user sessions.

I have an ask_login.php page where I simply ask the user to enter his/her login details through a form.

Once he/she logs in I want to save his/her login session until he/she decides to logout.

But for some reason I can’t manage to save the user session, because I tried to print the username while navigating to another part of the site and the username doesn’t show up.

Here’s my code:

ask_login.php

    <h1>Login</h1>
    <form method="post" action="login.php">
    <p><input type="text" name="username" value="" placeholder="UserID"></p>
    <p><input type="password" name="password" value=""  placeholder="Password"></p>
    <input type="submit" name="commit" value="Login"></p>
    </form>

login.php

    <?php

session_start();

$username = $_POST['username'];
$password = $_POST['password'];

if(!empty($username) && !empty($password)) {
    $queryget = mysql_query("SELECT * FROM users WHERE id='$username' AND password='$password'");
        $numrows = mysql_num_rows($queryget);

        if($numrows != 0) {

                    //save session
            $_SESSION['id'] = $username;

        } else if (empty($username) || empty($password)){
            echo "Wrong username/password";
            echo "<script>alert('Username/Password are incorrect');</script>";
            echo "<script language='javascript'>window.location = 'ask_login.php';</script>";
        } 
}

?>

logout.php

<?php
session_start();
session_destroy();
header('Location: index.php');
?>

You only store the user id in the $_SESSION:

$_SESSION['username'] = $username;

Furthermore you will need to call session_start() every time. Therefore most PHP programmers use a header.php file that is called each time and does some bookkeeping in the beginning of a page.

You should start the session on every page where you want to use the array $_SESSION.
To test wither or not you have the information stored in the session, do this:

session_start();
print_r($_SESSION);