Composer is an open source PHP dependency management tool designed to facilitate distribution and maintenance of PHP packages as standalone application components. It has dramatically changed the PHP ecosystem by creating component-based applications and frameworks that are the foundation of modern PHP development.
Requirements are declared in a project-level JSON file, which Composer then uses to evaluate which package versions best match the dependencies of the application. This assessment takes into account nested dependencies and system requirements, if any.
Packagist is a common repository for Composer, containing an open source PHP library that is freely available through Composer. The advanced version of the service provides hosting of private packages, making it possible to use Composer on closed source projects.
For more information on this topic, you can refer to our guide on how to install and use Composer in a PHP environment.