Rob Allen has a quick post today showing how to use Composer to globally install tools and libraries on your system.
The Composer package manager along with the Packagist repository site is quickly becoming the defacto PHP package management system. One feature I found out about recently is that you can install packages globally rather than locally into your project. I think that this is most useful for development tools, such as PHPUnit which are then available everywhere.
He includes the command syntax you'll need to do the global install, showing an example with the popular PHP unit testing tool PHPUnit. There's also a "global update" command you can use to update these packages (or add more) later on too.