http://www.tara.tcd.ie/handle/2262/77562 - "Design and implementation of an ahead-of-time compiler for PHP" is the PhD thesis in question.

A google tech talk related to this work is on youtube https://www.youtube.com/watch?v=kKySEUrP7LA

You can find multiple other papers on google scholar where Paul was an author all related to programming languages, compilers etc.

Some people are much better with theory than the application of that theory. I've met a lot of theory people that just care about implementing something that does something to get the result they want rather than the nitty gritty of making things efficient.