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PHP: How to check if user is already logged in and otherwise redirect to login page

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I am new to PHP and am struggling with the following:

I have a page where I want to check if someone is a registered user before letting them see the content of the site. So my thought was that in my header file (which is referenced on all single pages via require_once("includes/header.php");) I can check on that and redirect them to a login page (login.php) if they have not logged yet.

So here is everything that I have in my header: 

<!DOCTYPE html>
<html>
    <head>
        <?php 
            define("someUnguessableVariable", "anotherUnguessableVariable");
            session_start();
            if(!(isset($_SESSION['login']) && $_SESSION['login'] != '')){
                header ("Location: login.php");
            }

            include "system/config.php";

            $pageURL = basename($_SERVER["REQUEST_URI"]);
            $pageName = pathinfo(parse_url($pageURL, PHP_URL_PATH), PATHINFO_FILENAME); 

            $selectedLang = $_GET["lang"];
                if(!isset($selectedLang)){
                    $selectedLang = "de";
                }
            $langURL = "?lang=" . $selectedLang;

            $conn = new mysqli($dbServer, $dbUser, $dbPass, $dbName);
            $conn->set_charset("utf8");
            if($conn->connect_error){
                die("Connection failed: " . $conn->connect_error);
            } 
            // fetch main translations
            $location = "%main%";
            $stmt = $conn->prepare("SELECT tID, " . $selectedLang . " FROM TranslationsMain WHERE location LIKE ? ORDER BY tID");
            $stmt->bind_param("s", $location);
            $stmt->execute();
            $result = $stmt->get_result();  
            while($arrTranslations = $result->fetch_assoc()){
                $trans[] = array("ID" => $arrTranslations["tID"], "trans" => $arrTranslations[$selectedLang]);
            }
            $conn->close();

            // get main translations by ID
            function fetchTransMain($trans, $itemID){
                foreach($trans as $key => $val){
                    if($val["ID"] == $itemID){
                        return $val["trans"];
                    }
                }
            }
        ?>

        <meta charset="utf-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta name="author" content="Some author" />
        <meta name="description" content="Created: 2015-06" />

        <base href="http://www.myurl.de" target="_self" />

        <title>Some title</title>

        <!-- CSS -->        
        <link rel="stylesheet" type="text/css" href="includes/styles.css" />
        <!-- CSS - Font Awesome -->
        <link rel="stylesheet" type="text/css" href="http://maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css" />

        <!-- include favicon -->
        <link rel="shortcut icon" href="images/favicon/favicon.ico" type="image/x-icon" />
        <link rel="icon" href="images/favicon/favicon.png" type="image/png" />
        <link rel="icon" sizes="32x32" href="images/favicon/favicon-32.png" type="image/png" />
        <link rel="icon" sizes="64x64" href="images/favicon/favicon-64.png" type="image/png" />
        <link rel="icon" sizes="96x96" href="images/favicon/favicon-96.png" type="image/png" />
        <link rel="icon" sizes="196x196" href="images/favicon/favicon-196.png" type="image/png" />
        <link rel="apple-touch-icon" sizes="152x152" href="images/favicon/apple-touch-icon.png" />
        <link rel="apple-touch-icon" sizes="60x60" href="images/favicon/apple-touch-icon-60x60.png" />
        <link rel="apple-touch-icon" sizes="76x76" href="images/favicon/apple-touch-icon-76x76.png" />
        <link rel="apple-touch-icon" sizes="114x114" href="images/favicon/apple-touch-icon-114x114.png" />
        <link rel="apple-touch-icon" sizes="120x120" href="images/favicon/apple-touch-icon-120x120.png" />
        <link rel="apple-touch-icon" sizes="144x144" href="images/favicon/apple-touch-icon-144x144.png" />
        <meta name="msapplication-TileImage" content="favicon-144.png" />
        <meta name="msapplication-TileColor" content="#ffffff" />

        <script>
            var baseURL = '<?php echo $baseURL; ?>';
            var pageURL = '<?php echo $pageURL; ?>';
            var pageName = '<?php echo $pageName; ?>';
            var selectedLang = '<?php echo $selectedLang; ?>';
        </script>
    </head>   
    <body>

Now this is not working and I think I am probably missing a couple of things but I couldn't find a good tutorial or guideline on that. Also, I am not sure if there is anything else I need to do in order to start and set up the session.

Can someone help me with this ?

Note:
This is only about checking if a user is already logged in since all the actual user registration and verification is done on the separate login page and for this I already have the code working.

Update: Enabling error messages returns the following errors: 

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /homepages/21/d580042014/htdocs/index.php:2) in /homepages/21/d580042014/htdocs/includes/header.php on line 9

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /homepages/21/d580042014/htdocs/index.php:2) in /homepages/21/d580042014/htdocs/includes/header.php on line 9
array(0) { } 
Warning: Cannot modify header information - headers already sent by (output started at /homepages/21/d580042014/htdocs/index.php:2) in /homepages/21/d580042014/htdocs/includes/header.php on line 12

Notice: Undefined index: lang in /homepages/21/d580042014/htdocs/includes/header.php on line 18

Update:
As per the comments I now posted everything that's currently in the header.

Many thanks in advance.

php session redirect authentication session-variables
Original Q&A
4

There are 4 best solutions below

15
On BEST ANSWER

Update: The question has been resolved in chat.

As per your edit, change this block:

<!DOCTYPE html>
<html>
    <head>
        <?php 
            define("someUnguessableVariable", "anotherUnguessableVariable");
            session_start();
            if(!(isset($_SESSION['login']) && $_SESSION['login'] != '')){
                header ("Location: login.php");
            }

to:

<?php 
session_start();
?>

<!DOCTYPE html>
<html>
    <head>
        <?php 
            define("someUnguessableVariable", "anotherUnguessableVariable");
            
            if(!isset($_SESSION['login']) && $_SESSION['login'] != ''){
                header ("Location: login.php");
                exit; // stop further executing, very important
            }
  • Follow the same structure for starting the session in all your files using sessions.
  • Make sure that your file does not have a byte order mark (BOM).
  • No space before <?php etc. this has already been established in comments.

Using a code editor such as Notepad++ https://notepad-plus-plus.org/ and to save it as UTF-8 without BOM which will ensure there is no byte order mark.

Also, using the new method for your sessions array check.

if(!isset($_SESSION['login']) && $_SESSION['login'] != ''){

Also check to see that none of your included/required files have the same issues, including login.php.

Footnotes:

Inside Notepad++'s dropdown menu, you will see

  • Encoding. It will show you what the present file's encoding is set to.

If it does show a byte order mark, follow these steps:

  1. Click on "Encoding".
  2. Convert to UTF-8 without BOM
  3. Save the file.
  • Do this for all your files.

Reference(s):

Sidenote:

You should change $stmt->execute(); to

if(!$stmt->execute()){
    trigger_error("there was an error....".$conn->error, E_USER_WARNING);
}
  • It's better to catch possible errors in your query.
5
On

Put the user profile in session variables in PHP script you call after the login page

$_SESSION['user_id'] = $row["user_id"];
$_SESSION['profile_id'] = $row["profile_id"];
$_SESSION['name'] = $row["name"];
$_SESSION['surname'] = $row["surname"];
$_SESSION['application_auth'] = $row["application_auth"];

Put the following code at the top of each page you want to protect by not valid users

<?php 
include("sessionCheck.php");
?>

The sessionCheck script In this case I also check if the user is entitled to view the specific page using the profile_id but you can remove it 

<?php
    session_start();
    if(!IsSet($_SESSION['user_id']) or $_SESSION['profile_id'] !=1)
    {
        header("location: http://www.yourdomain.com/login.php?message=Invalid user");
    }   
?>
6
On

You need to move 

session_start();
if((!isset($_SESSION['login']) && $_SESSION['login'] != '')){
    header ("Location: login.php");
}

to the top of the scrip and move ! inside the bracket.

5
On

The following portion should go before include "system/config.php"; because it seems like this file is outputting and also the fact that the following code is not dependant on any other data.

session_start();
if(!(isset($_SESSION['login']) && $_SESSION['login'] != '')){
    header ("Location: login.php");
}

Second:

if(!(isset($_SESSION['login']) && $_SESSION['login'] != '')){

Should probably be: if(!isset($_SESSION['login']) && $_SESSION['login'] == ''){

meaning == '' and not != ''

A user not logged in will most likely (depending on your code) not have a value set so your code will check to see if it's not blank so only logged in users will get forwarded.

Hence:

<?php
define("someUnguessableValue", "anotherUnguessableValue");
session_start();
if(!isset($_SESSION['login']) && $_SESSION['login'] == ''){
    header("location:login.php");
}

include "system/config.php";
//.... the rest
?>
<!DOCTYPE html>
<html>
    <head>

If it's not forwarding, then $_SESSION['login'] is most likely set, you need to clear your cookies for that domain.

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