Composer is a tool for dependency management in PHP. It allows you to declare the libraries your project depends on and it will manage (install/update) them for you.
Composer is not a package manager in the same sense as Yum or Apt are. 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 does not install anything globally. Thus, it is a dependency manager. It does however support a "global" project for convenience via the global command.
- You have a project that depends on a number of libraries.
- Some of those libraries depend on other libraries.
- Enables you to declare the libraries you depend on.
- Finds out which versions of which packages can and need to be installed, and installs them (meaning it downloads them into your project).