I can help with the react lib. I have interest in that lib and already started with one of those libs -- php-react-promise.
Regarding Guzzle v3, I will need to find time to look into the current test failures which make the package unavailable in rawhide. It is available in Fedora <= 25 and EPEL 6/7 though:
$ dnf repoquery --provides php-guzzle-Guzzle
Last metadata expiration check: 0:34:35 ago on Mon Jan 16 13:59:48 2017.
php-composer(guzzle/batch) = 3.9.3
php-composer(guzzle/cache) = 3.9.3
php-composer(guzzle/common) = 3.9.3php-composer(guzzle/guzzle) = 3.9.3
php-composer(guzzle/http) = 3.9.3php-composer(guzzle/inflection) = 3.9.3
php-composer(guzzle/iterator) = 3.9.3
php-composer(guzzle/log) = 3.9.3
php-composer(guzzle/parser) = 3.9.3php-composer(guzzle/plugin) = 3.9.3
php-composer(guzzle/plugin-async) = 3.9.3
php-composer(guzzle/plugin-backoff) = 3.9.3
php-composer(guzzle/plugin-cache) = 3.9.3
php-composer(guzzle/plugin-cookie) = 3.9.3
php-composer(guzzle/plugin-curlauth) = 3.9.3
php-composer(guzzle/plugin-error-response) = 3.9.3
php-composer(guzzle/plugin-history) = 3.9.3
php-composer(guzzle/plugin-log) = 3.9.3
php-composer(guzzle/plugin-md5) = 3.9.3
php-composer(guzzle/plugin-mock) = 3.9.3
php-composer(guzzle/plugin-oauth) = 3.9.3
php-composer(guzzle/service) = 3.9.3
php-composer(guzzle/stream) = 3.9.3php-guzzle = 3.9.3-9.fc25
php-guzzle-Guzzle = 3.9.3-9.fc25
php-guzzle-guzzle = 3.9.3-9.fc25
php-pear(
guzzlephp.org/pear/Guzzle) = 3.9.3