从file_get_contents请求中获取IP

I have two pages, one sending a file_get_contents request:

$postdata = http_build_query( array('install' => 'true', 'url' => $_SERVER['SERVER_NAME'], 'key' => 'XXXXXXXXXXXX') );

$opts = array('http' =>
    array(
        'method'  => 'POST',
        'header'  => 'Content-type: application/x-www-form-urlencoded',
        'content' => $postdata
    )
);

$context  = stream_context_create($opts);
$registration_key = file_get_contents('http://example.com/register.php', false, $context);

And a page that receives the call (http://example.com/register.php) and I am trying to grab the IP of where the request is coming from to no avail. I have tried:

$_SERVER['HTTP_CLIENT_IP']

$_SERVER['HTTP_VIA']

$_SERVER['HTTP_X_FORWARDED_FOR']

$_SERVER['REMOTE_ADDR'] (IP, but not the IP of the computer sending OR receiving the request)

Any idea as to where I can find more info on the computer sending the request?

Thanks in advance!

Only if you will pass IP in request:

$registration_key = file_get_contents('http://example.com/register.php?ip='.$_SERVER['REMOTE_ADDR'], false, $context);