Deployer is a deployment tool written in PHP.
We have used Phing for many years to not only do our builds and general project tasks, but also to do our deployments. A few years ago, we started to look for alternatives as Phing has it's limits when it comes to ease-of-use and entry barrier for new programmers.
One of the obvious choices was Capistrano. While we like Capistrano very much, we felt a PHP based solution would be preferable: while a few of our developers know Ruby, all our programmers know PHP. It would be better to make this tooling improvable by and understandable for everyone (any team should be able to improve on their tooling on their own, think DevOps).
We were pretty happy when we found that Deployer is comparable to Capistrano feature-wise and fits our requirements very well. We have since then adopted it on some of our projects and will roll it out to more PHP projects. When the testing in a broad range of PHP projects succeeds, we will likely adopt it as our standard for deployments.