Login with Facebook ( In 20 lines of PHP code )

login.php

<?php
// Set $apiVersion, $apiSecret, $redirectUrl, and $scope etc here.
if($_GET['logout']!=""){                                                                                                                              
    session_destroy();
    exit("Logged out");
}
if($_GET['code']!=""){
    $accessData = json_decode(file_get_contents("https://graph.facebook.com/".$apiVersion."/oauth/access_token?client_id=".$appId."&redirect_uri=".$redirectUrl."&client_secret=".$appSecret."&code=".trim($_GET['code'])),true);
    $userInfo = json_decode(file_get_contents("https://graph.facebook.com/v2.7/me?fields=email,id,name&access_token=".$accessData['access_token']),true);
    $_SESSION['access'] = $userInfo;
}else{                                                                                                                                                        
        if($_SESSION['access']['id']!=""){
            echo 'Welcome '.$_SESSION['access']['name'].' <a href="?logout=true">Logout</a>';
        }else{
            ?>  
                <a href="https://www.facebook.com/dialog/oauth?client_id=<?=$appId?>&redirect_uri=<?=$redirectUrl?>&scope=<?=$scope?>">Login with Facebook</a>
            <?php
        }   
}
?>