Which version of Behat (Guzzle) would be okay to use on a RHEL server with PHP 5.4? On a Red Hat Enterprise Linux Server release 7.1 (Maipo) development server using [Behat Drupal Extension][1], a RHEL 7 / PHP 5.4 installation. When I try to run behat from the command line on a dev server, I get the following error:

$ ./vendor/bin/behat



  Feature: First
        Playing with features and exploring
  
  @api
  
  Scenario: Run cron                                             # features/first.feature:6
      PHP Fatal error:  Call to undefined function GuzzleHttp\Handler\curl_reset() 
  
  in /var/www/html/sites/default/behat-tests/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php
  on line 78


From answers below, it appears the the version of Guzzle bundled with this requires 5.5 or greater. 

  guzzlehttp/guzzle               6.1.0   Guzzle is a PHP HTTP client library
  guzzlehttp/promises             1.0.3   Guzzle promises library
  guzzlehttp/psr7                 1.2.0   PSR-7 message implementation


My question: what version of Guzzle should I use in order to be able to run test on a 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.