Which version of Behat (Guzzle) would be okay to use on a RHEL server with PHP 5.4?
Call to undefined function GuzzleHttp\Handler\curl_reset()


Means PHP is trying to find the function curl_reset() in the namespace GuzzleHttp\Handler.

It does that because it can't find it in the global namespace (where it should be). curl_reset() was only introduced in PHP 5.5, and you have version 5.4, so your version just isn't compatible with Guzzle 5.

Guzzle isn't explicitly listed as a dependency in the Behat extension's composer.json file, so one of the other libraries must depend on it. I'm not sure it would even be worth the time trying to unpick that, upgrading php would be much quicker.