Fedora Core 5 ships with SELinux. Even having installed all the requisite Apache, PHP and Libcurl packages, cURL may not work correctly. It's not a package dependency conflict, or a bug - just an overprotective security manager keeping things safe.

I encountered this behaviour when trying to send a post request from my server to another. It manifested itself as a 'Failed to connect to XXX.XXX.XXX.XXX (ip): Permission denied'.

By default 'Allow HTTPD scripts and modules to connect to the network' is unticked in 'system-config-securitylevel' ('SELinux' tab). Relaxing this security restriction solved the problem.