Composer is an open source dependency management tool for PHP, created primarily to facilitate the distribution and maintenance of PHP packages as individual application components. It has dramatically changed the PHP ecosystem by creating the basis for modern PHP development, with component-based applications and frameworks.

Requirements are declared in a project-level JSON file, which Composer then uses to evaluate which package versions are the best match for the application dependencies. The evaluation will take into consideration nested dependencies and system requirements, if any.

Packagist, the public Composer repository, contains an expressive collection of open source PHP libraries that are made available for free through Composer. A premium version of the service offers hosting for private packages, making it possible to also use Composer on closed source projects.

To learn more about this subject, you can refer to our guides on how to install and use Composer within PHP environments.