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>


/home/delhioia/public_html/index.php
<?php
session_start();
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head><title>Birth, Marriage, Domicile Certificates, Passport, Voter ID Card, Aadhar/UIDAI Card, Driving Licence in Delhi</title>
<meta name="description" content="Find information, how to apply for Birth, Marriage, Domicile Certificates, Passport, Voter ID Card, Aadhar/UIDAI Card, Driving Licence issued by Delhi Govt.">
<meta name="keywords" content="birth certificate, marriage, domicile certificate, passport, voter ID card, aadhar/UIDAI card, driving licence">
<meta http-equiv="content-type" content="text/html; charset=iso-8859-1">
<meta name="google-site-verification" content="dZ_1PeO1D2NtRj0IamyazEPbRyAsOrpY33IHVI5TbyU" />
<link rel="stylesheet" type="text/css" href="style/style.css">
<script type="text/javascript" src="js/jquery-latest.js"></script>
<script type="text/javascript" src="js/jquery.min.js"></script>
</head>
<style>
li{padding-bottom:10px;}
</style>
<body>