Question

I am having trouble with my code.It is supposed to redirect to a page if the login is okay.I am getting a successfully login but I am getting a error with the header forward.Here is the line which the problem is occurring:

header("Location: members.php");

Here is the error message:
Warning: Cannot modify header information – headers already sent

Here is my full code for the page if it helps:

<?php 
    // Connects to your Database 
    include("dbconnect.php");
    mysql_select_db("maxgee_close2");
    //Checks if there is a login cookie

    if(isset($_COOKIE['ID_my_site']))
     //if there is, it logs you in and directes you to the members page
    { 
        $username = $_COOKIE['ID_my_site']; 
        $password = $_COOKIE['Key_my_site'];
        $check = mysql_query("SELECT * FROM users WHERE username = '$username'")or die(mysql_error());
        while($info = mysql_fetch_array( $check )) 
        {
            if ($pass != $info['password'])
            {
            }
            else
            {
                header("Location: members.php");
            }
        }
    }
    //if the login form is submitted 
    if (isset($_POST['submit'])) { // if form has been submitted
     // makes sure they filled it in
        if(!$_POST['username'] | !$_POST['password']) {
            die('You did not fill in a required field.');
        }
        // checks it against the database

        if (!get_magic_quotes_gpc()) {
            $_POST['email'] = addslashes($_POST['email']);
        }
        $check = mysql_query("SELECT * FROM users WHERE username = '".$_POST['username']."'")or die(mysql_error());

        //Gives error if user dosen't exist
        $check2 = mysql_num_rows($check);
        if ($check2 == 0) {
            die('That user does not exist in our database. <a href=add.php>Click Here to Register</a>');
        }
        while($info = mysql_fetch_array( $check ))  
        {
            $_POST['password'] = stripslashes($_POST['password']);

            $info['password'] = stripslashes($info['password']);

            $_POST['password'] = md5($_POST['password']);

            //gives error if the password is wrong

            if ($_POST['password'] != $info['password']) {
                die('Incorrect password, please try again.');
            }
            else 
            { 
                // if login is ok then we add a cookie 
                $_POST['username'] = stripslashes($_POST['username']); 
                $hour = time() + 3600; 

              //then redirect them to the members area and the line with the error
              header("Location: members.php");
            }
        } 
      }
      else
      { 
        // if they are not logged in
         ?>
         <form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">
         <h1>Login</h1>
         Username:
        <input type="text" name="username" maxlength="40"> 
        Password:
        <input type="password" name="password" maxlength="50"> 
        <input type="submit" name="submit" value="Login">
        </form> 
    <?php 
     } 
     include("topsite.php");
    ?> 

Answer 1

You have written

$password = $_COOKIE['Key_my_site'];

But you are comparing $pass with $info[‘password’] and it should be

    if ($password != $info['password']){

    } else {

        header("Location: members.php");
    }

You can modify your query in a better way by writing

$check = mysql_query("SELECT * FROM users WHERE username = '$username' AND password = '$password'")or die(mysql_error());

if(mysql_num_rows($check) == 0) header("Location: members.php");