How to fix failed communications with Payment Gateways in PHP using Curl/SOAP

What is the timeout setting you have set?  You might want to try adding a little more time on that and see if that helps.

Timeout settings where?  php.ini, the gateway api, ...

How do I diagnose failed communications on linux, is there a log file?

Look into this:  http://www.php.net/manual/en/function.set-time-limit.php

And check this out:  http://stackoverflow.com/questions/2582057/setting-curls-timeout-in-php

The first one is about setting it up in php.ini or setting it in code.  The second one is regarding the CURL timeout specifically.

Is a 1-2% failure rate for communicating with other servers acceptable?

"Acceptable" is a pretty broad term.  Some questions to help run this to ground...

Are you on a shared server?

What PHP functions are causing the "communications errors?"

Are you using cURL error reporting?

Have you tried fsockopen() to speak with the APIs?

using curl

Should I always set CURLOPT_CONNECTTIMEOUT and CURLOPT_TIMEOUT?

Are system defaults set somewhere for CURLOPT_CONNECTTIMEOUT and CURLOPT_TIMEOUT?

Does /etc/php5/apache2/phi.ini->max_execution_time affect CURLOPT_CONNECTTIMEOUT and CURLOPT_TIMEOUT or does max_execution_time get suspeneded with a call is made to an external system via curl?

is there a difference with curl in apache (browser) vs cli (command line cron jobs?)