I have been doing a lot of benchmarking lately with regards to what is the most efficient way to program PHP. PHP Benchmark is a site that is dedicated to that single task.
This site is in the early stages but looks very promising and deals with some of the major assumptions that programmers have about PHP, including the single and double quotes issue. I would say that most of the tests are only run a few times (around 1000) and so this doesn't give a full representative picture, but I look forward to seeing more from this site in the future.
Here are some links to my own benchmarking tests on #! code