Dependency Manager for PHP.
Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.
Dependency management
Composer is not a package manager. Yes, it deals with "packages" or libraries, but it manages them on a per-project basis, installing them in a directory (e.g. vendor) inside your project. By default it will never install anything globally. Thus, it is a dependency manager.
This idea is not new and Composer is strongly inspired by node’s npm and ruby’s bundler. But there has not been such a tool for PHP.
The problem that Composer solves is this:
a) You have a project that depends on a number of libraries.
b) Some of those libraries depend on other libraries .
c) You declare the things you depend on
d) Composer finds out which versions of which packages need to be installed, and installs them (meaning it downloads them into your project).