Composer is a tool for PHP dependencies management. It allows you to declare the dependent libraries your project needs and it will install them in your project for you. Composer is not a package manager. Though dealing with "packages" or libraries, it manages them on a per-project basis, installing them in a directory inside your project.
The Composer helps in the following scenarios:
- A project depending on a number of libraries.
- Some of those libraries depend on other libraries.
- You declare the things you depend on.
- Composer finds out which versions of which packages need to be installed, and installs them (meaning it downloads them into your project).
For more information on Composer, see https://getcomposer.org/.