Peter Petermann has an interesting post he's added to his site describing a lesser known feature of the Composer package manager: virtual package support.

A few days ago i stumbled over a “virtual package” on packagist – and found it to be a feature that i was actually missing in composer. Turns out, composer can do it, its just not so well documented. So what is this about? Virtual packages allow you to have a more loose dependency. Rather than depending on a specific package, you depend on a virtual one, which can be fulfilled by all packages that provide the virtual one.

He includes a few examples to help illustrate the point of using virtual packages. The first describes an application that wants to use the PSR-4 logger structure but depends on "log-implementation" (a virtual package) rather than the "psr/log" package. The key is in using the "provide" keyword in the Composer configuration. His other two examples expand on this a bit, one showing the use of the "provide" keyword to define the relationship and the other of an actual application making use of this package.