I wrote a PHP script that needs to connect to port 53 on UDP and on
some (shared) servers it doesn't seem to be working. This makes me
currious - is it possible that these servers connections on specific
ports has been disallowed? If so, how would one go about doing this?

Also, how might I go about detecting it?

>I wrote a PHP script that needs to connect to port 53 on UDP and on[color=blue]
>some (shared) servers it doesn't seem to be working. This makes me[/color]

Port 53 on the DESTINATION system, port 53 on the LOCAL system, or
both? Are you sure the destination system is running a server on
port 53?

If the local system is running its own DNS server (may be a good
idea for a hosting web server) on port 53, you can't bind to port
53 on the local system because the nameserver is already using it.
Also, binding to low-numbered ports ( < 1024 ) may require root
privilege. But to talk to port 53 you don't need port 53 on your
end.
[color=blue]
>currious - is it possible that these servers connections on specific
>ports has been disallowed? If so, how would one go about doing this?[/color]

Firewalls, either on the host itself (Linux and FreeBSD have things
like ipf and ipfw in the kernel if it's built that way), or it could
be a nearby Cisco router). Port 53 isn't something normally blocked,
though. It's sometimes the *ONLY* UDP let through.
[color=blue]
>Also, how might I go about detecting it?[/color]

I think you just did. Try to send a packet and see what comes back
(ICMP port unreachable, ICMP host unreachable, ICMP prohibited, nothing,
or a response) with something like tcpdump.

Gordon L. Burditt