cURL not returning anything

I copied one of my PHP applications to a new server that is within a firewall. The new server is running PHP 4.0.6 and cURL 7.8.1. I use cURL to connect to a gateway and process credit card transactions. In any case, if I run curl from the command line with all the required POST fields, I get an immediate response from the gateway and it works perfectly. HOWEVER, if I use PHP+cURL to run the EXACT same test transaction, I don't get any response back. It seems as though the fetching portion works. I even tried the sample cURL program off of PHP.net's manual, and it seemed to be fetching the sample HTML page, but nothing came back. It's like the fetching process works, but there's a problem returning the contents to either a file or a variable. But there are no errors reported by PHP. Help! - Jonathan