It appears that setting CURLOPT_FILE before setting CURLOPT_RETURNTRANSFER doesn't work, presumably because CURLOPT_FILE depends on CURLOPT_RETURNTRANSFER being set.
So do this:
<?php
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FILE, $fp);
?>
not this:
<?php
curl_setopt($ch, CURLOPT_FILE, $fp);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
?>