On the StarTutorial.com site they've posted a tutorial for those that might be newer to the PHP language and ecosystem around it covering the use of Composer. In this latest tutorial they cover some of the basics of using this package manager to pull in and use dependencies in your PHP application.

In general, a block of code forms a method, a group of methods forms a class and a set of classes form a package. A reusable package can be dropped into any project and be used without any need to add functionality to it. A package exposes APIs for clients to achieve a single goal. Packages help our applications achieve DRY (Don't Repeat Yourself), a principle of software development, which reduces repetition of information of all kinds.

[...] In most cases, packages have dependencies. [...] We do need a package manager, a package manager that can solve all of these dependency headaches for us.

The tutorial then goes on to compare two of the main package managers in PHP: Composer and PEAR. It then walks through the installation of Composer and how to use it to pull in a package (in this case the "Faker" package for generating fake user data). It also explains the different commands and configuration files used in working with Composer.