Targeting a ubiquitous virtual machine is all the rage these days. All the cool kids are doing it. On the client-side, everything is the browser and/or flash with silverlight “coming soon”. On the server things are not much different with every man and his language running on the JVM or .NET. Of course HaXe follows this very sensible tradition and runs on both flash and the browser (and more) on the client and, erm, Neko on the server.
So, why does HaXe target Neko? Well, it is common knowledge that on the server, it doesn’t really matter what you run. As long as you are web enabled, no-one cares. Actually, that is only true for the small number of people who run their own server. A far larger group of people write code that runs on a free host.
PHP isn’t usually thought of as a virtual machine but it is very common on free hosts. If you are writing your own compiler, you can target whichever runtime you like including PHP. I know Neko has many advantages over PHP including a huge speed advantage but ubiquity counts for lot. I suspect a PHP backend for HaXe would boost the number of potential users through the roof.