Alexandria is a library of PHP code that will continue to grow as we find more time to drop components into it. Right now there's not too much stuff in there, but the main pieces that are likely to be of immediate use are a PHP smtp_injector extension and a supporting PHP 5 class that should prove useful to folks that generate mailings from PHP. You can read more about those in the documentation.
I want to stress that Alexandria is not a framework. Each piece is relatively self contained and tries to avoid imposing a particular programming paradigm on the apps that use it.
Labs contains more than just a PHP library; we have a number of other projects, and more will make appearances over time.