I integrated Login with FB and Twitter; but I'm getting error when I click Login in Facebook link

check at http://delhicertificates.in/

Warning: Cannot modify header information - headers already sent by (output started at /home/delhioia/public_html/index.php:7) in /home/delhioia/public_html/facebook/index.php on line 12
/home/delhioia/public_html/facebook/index.php
<?php
//Always place this code at the top of the Page
if (isset($_SESSION['id'])) {
    // Redirection to login page twitter or facebook
    header("location: home.php");
}
if (array_key_exists("login", $_GET)) {
    $oauth_provider = $_GET['oauth_provider'];
    if ($oauth_provider == 'twitter') {
        header("Location: login-twitter.php");
    } else if ($oauth_provider == 'facebook') {
        header("Location: login-facebook.php");
    }
}
?>
<div style='padding-top:5px;'>
<a href="?login&oauth_provider=twitter"><img src="../img/twitter_signin.png"></a><a href="?login&oauth_provider=facebook"><img src="../img/facebook_signin.png"></a>
</div>

Have you tried putting
session_start()
at the top of the page?

as fobos said
put session_start() function on top of your page

session_start() is already in index.php (at root); I'm using include() to include this file

Please check if there is any "echo" statement above any header.If there is then it will give this error or there might be some spaces.
Check this link.

try placing this at the top of your page:

<? ob_start(); ?>

then at the bottom of the page place this line of code:

<? ob_flush(); ?>

I assume the /home/delhioia/public_html/facebook/index.php file is being included from /home/delhioia/public_html/index.php, right?

The error message says that /home/delhioia/public_html/index.php is sending output on line 7, probably before inclusion of the /home/delhioia/public_html/facebook/index.php that is why the error, I presume (provided that my assumptions are right).

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