Composer reads `composer.json`, resolves versions from Packagist, and installs packages into `vendor/` with an autoloader — `composer require symfony/http-foundation` adds a library and its dependencies. It modernized PHP the way npmNode Package Manager — the default registry and CLI for installing JavaScript and TypeScript libraries. modernized NodeNode.js — JavaScript on the server via an event-driven runtime built on V8.. Example: declare PSR-4 autoloading and run `composer install` in CI for reproducible deploys.

Composer brought semver dependency management to PHP after years of PEAR and manual includes.

It enabled a modern PHP ecosystem with Laravel, SymfonyA PHP framework and component library — enterprise web apps and APIs across Europe., and thousands of reusable packages.