isset() didn't work after login with $_SESSION set to valid

问题

I have this loginform.php with this part of code (this was called from index.php with a login form):

include("config.php");
if(isset($_POST['submit']))
{
    $username = $_POST['username'];
    $password = $_POST['password'];

    if($username == "" || $password == "")
    {
        echo "Either username or password field is empty.";
        echo "<br/>";
        echo "<a href='index.php'>Go back</a>";
    }
    else
    {
        $result = mysql_query("select * from users where user_username='$username' and user_password='$password'",$conn)
        or die("Could not execute the select query.");

        $row = mysql_fetch_assoc($result);

        if(is_array($row) && !empty($row))
        {
            $validuser = $row['username'];
            $_SESSION['valid'] = $validuser;
        }
        else
        {
            echo "Username and password do not match.";
            echo "<br/>";
            echo "<a href='index.php'>Go back</a>";
        }

        if(isset($_SESSION['valid']))
        {
            header("Location:index.php?");
        }
    }
}

but after that it should've shown again the index.php without the login form

index.php had this code to display the user after successful login:

    <?php
if(isset($_SESSION['valid']))
{   
    include("config.php");
    $result = mysql_query("select * from users",$conn);
    echo "Welcome ".$_SESSION['valid']. "! <a href='logout.php'>Logout</a><br/>";
}
else
{
    require('./loginform.php');
}
?>

with this code the login form do not showed up but instead not showing anything. It should display the user that had logged in. I dont know what I missed. Im a newbie at php. Please help. sorry, please assumed that there is a session start at all file.

回答1:

You have to add session_start() before checking the existence of the session variable.

<?php
session_start();
if(isset($_SESSION['valid']))
....

session_start() creates a session or resumes the current one.

来源：https://stackoverflow.com/questions/19476873/isset-didnt-work-after-login-with-session-set-to-valid