How to get the real URL after file_get_contents if redirection happens?

You might make a request with cURL instead of file_get_contents().

Something like this should work...

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HEADER, TRUE);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, FALSE);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
$a = curl_exec($ch);
if(preg_match('#Location: (.*)#', $a, $r))
 $l = trim($r[1]);

Source

If you need to use file_get_contents() instead of curl, don't follow redirects automatically:

$context = stream_context_create(
    array(
        'http' => array(
            'follow_location' => false
        )
    )
);

$html = file_get_contents('http://www.example.com/', false, $context);

var_dump($http_response_header);

Answer inspired by: How do I ignore a moved-header with file_get_contents in PHP?