Slimframework: a nice and cool microframework

Symfony: a bigger framework which is filled with great and reusable components

Guzzle: a simple and easy to use HTTP client

PHPUnit: a framework for unitary testing

Behat: a framework for Behavior-Driven Development

PHPCS/CBF: code sniffer and code beautifier

Faker: fake data generator

Psysh: a runtime developer console (CLI) full of amazing features

Composer: dependency management and other useful features

Twig: template engine