This change was brought about by the fact that our own HTTP client is dependent on a PHP extension that (at least currently) doesn't work with PHP 5.5 and which is not under any particular active development.
Guzzle, on the other hand, is not dependent on any PHP extensions and is continually being improved and updated.
As the provider of a service that is heavily based around retrieving resources from the web for later testing, the HTTP client we use to do this is at the centre of everything that goes on.
Switching HTTP client libraries turned out to be a non-trivial task.
Guzzle handles error cases differently, presents a slightly different means for mocking HTTP responses in code tests and provides a slightly different means for retrying failed requests. None of this is bad, it's just different and takes time to apply correctly.
Did I mention that the HTTP client is at the centre of everything? Changing the HTTP client changes everything.
Well, maybe not everything, but lots:
All code tests that make use of mock (or pretend) HTTP responses had to be updated to use the slightly different means Guzzle uses for achieving this.
Everything had to be re-tested.
Areas of code that were changed that did not have code tests had to have tests created.
Minor issues spotted when creating and then running code tests had to be fixed.
At the end of this change, we now have a much more stable and future-proof system:
The code is more readily testable and has seen a large increase in the number of code tests in use, reducing the chance of inadvertently-introduced bugs getting released into the wild.
Everything works under PHP 5.3, 5.4 and 5.5.
Improved HTTP caching abilities present the option for further performance improvements.