Instead of using file_get_contents, you can use CURL
$ch = curl_init('http://imdbapi.org/?title=One+Piece&type=xml&plot=simple&mt=none&episode=0&aka=simple&release=simple');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
curl_close($ch);
Now $response shall contains your XML. And you can do something like
file_put_contents('filename.xml', $response);
make sure that filename.xml is writable