php ftp_nlist timeout in Heroku - php

Question

What
I'm trying to do a pretty basic ftp reading in Heroku.
Code:
$ftp = ftp_connect($config['domain']);
$login = ftp_login($ftp,$config['ftp_user'],$config['ftp_pass']);
ftp_pasv($ftp, true);
$contents = ftp_nlist($ftp, "/");
for ($i = 0 ; $i < count($contents) ; $i++)
echo "<li>" . substr($contents[$i],1) . "</li>";
ftp_close($ftp);
Heroku Error
2015-05-19T07:26:01.678102+00:00 heroku[router]: at=error code=H12 desc="Request timeout" method=GET path="/" host=xxx-ftp.herokuapp.com request_id=xxxxx-364a-48f8-8e2a-383affb0789f fwd="xx.12.8.106" dyno=web.1 connect=0ms service=30000ms status=503 bytes=0
2015-05-19T07:26:31.739770+00:00 app[web.1]: [Tue May 19 07:26:31.738789 2015] [proxy_fcgi:error] [pid 186:tid 140199224919808] (70007)The timeout specified has expired: [client 10.127.183.84:42907] AH01075: Error dispatching request to : (polling)
2015-05-19T07:26:31.853475+00:00 app[web.1]: [19-May-2015 07:26:31 UTC] PHP Warning: ftp_nlist(): php_connect_nonb() failed: Operation now in progress (115) in /app/list.php on line 26
Facts
The connection is ok (returning true)
Heroku is giving timeout only when ftp_nlist or ftp_rawlist is executed.
The login is ok
I've tried with and without PASIVE mode
Other basic commands as PWD are working ok
I've also tried by listing a more specific folder e.g: ftp_nlist($conn,"/MyFolder");
The amount of folder I'm trying to read and list is small.
The FTP server is responding in less than 2sec from other FTP clients
The same code in other server (not heroku) works fine.

Answer 1

This seems to work just fine for ftp.mozilla.org for instance:
$ftp = ftp_connect('ftp.mozilla.org');
$login_result = ftp_login($ftp, 'anonymous', '');
ftp_pasv($ftp, true);
var_dump(ftp_nlist($ftp, "/pub/mozilla.org/"));
Are you maybe hitting https://bugs.php.net/bug.php?id=55651 with your destination FTP behind NAT?