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.
A helper class, a small client wrapper or a set of validation rules often starts in one project and ends up copied into the next one. From that point the copies drift apart, a bug fixed in one of them stays open in the rest, and there is no version number to refer to when something changes.
Turning that code into a Composer package removes the copies. The requirements are modest, since a package is a directory containing a valid composer.json file, a Git repository hosting it, and a listing on Packagist that tells Composer where to find it.
