Composer is an application-level package manager for the PHP programming language that provides a standard format for managing dependencies of PHP software and required libraries. It manages them on a per-project basis, installing them in a directory (e.g. vendor) inside your project.
PurgeCSS is a tool to remove unused CSS. When you are building a website, you might decide to use a CSS framework like TailwindCSS, Bootstrap, MaterializeCSS, Foundation, etc... You will only use a small set of the framework but a lot of unused CSS styles will be included.
Redis is an open-source in-memory key-value data store. It can be used as a database, cache, message broker and supports various data structures such as Strings, Hashes, Lists, Sets, etc... Apart from its performance and flexibility, Redis stands out with its wide language support, high availability, and automatic partitioning.
Symfony is a PHP web application framework and a set of reusable PHP components/libraries. Symfony aims to speed up the creation and maintenance of web applications and to replace repetitive coding tasks. It's also aimed at building robust applications in an enterprise context, and aims to give developers full control over the configuration: from the directory structure to the foreign libraries, almost everything can be customized.