why doesn't my php redirect work?

Im writing a login script. My db.php doesnt echo/print anything so why doesnt header("Location: index.php"); redirect upon successful login? the login info is correct. I know i need to sanitize the input but th开发者_高级运维at is not a problem at the moment.

 <?php
        require('db.php');
        $username = $_POST['un'];
        $password = $_POST['pw'];
        $qry="SELECT uid FROM users WHERE name='$username' AND pass='".md5($password)."'";
        $result=mysql_query($qry);
        if(mysql_num_rows($result) == 1){
            session_regenerate_id();
            $user = mysql_fetch_assoc($result);
            $_SESSION['S_UID'] = $user['uid'];
            session_write_close();
            header("Location: index.php");
            exit();
        }else{
            echo "<center><form action='index.php' name='login'>Login failed! Please try again with correct username and password.<br><input type='submit' name='failed' value='Return to Login'></form></center>";
            exit();
        }
    ?>

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the function header will only work if no output has been sent by the script before the function is called. Check if any codes above has echoed something.

If not, check that the include files above do not have an extra space or newline after the closing "?>" tag. Otherwise this space or newline will be sent to the browser before your header.

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Are you sure no output is being produced anywhere in the script (or db.php)? Not even a few extraneous spaces ?

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Try using the full URL for example:

header("Location: http://www.mysite.com/index.php");

Some browsers (thanks IE) doesn't understand the 301 redirect code with uncompleted URI.

This header location behavior in PHP is a good tool, but easy to be implemented in the incorrect scenario...for this purpose i would consider using require() instead of header(location)

like:

if (!imLoggedIn()){
  require('loginForm.php');
  exit;
}