PlanOut PHP Port Code Review

This PR should provide the project maintainers over at [PlanOut](https://github.com/facebook/planout) an easy way to review adding the proposed PHP port contained within this repository as a submodule for the original project.

Timeline for merge:
- [ ] review proposed port in this branch
- [ ] merge changes back into this repository's master branch
- [ ] update submodule checkout commit in [PlanOut PR](https://github.com/facebook/planout/pull/31) to reflect changes
- [ ] merge port as submodule into main PlanOut repository

cc @eytan for review
Why not make _log() log to a file, like we do in the Python version?  It's then pretty easy for somebody who is first learning to use the software to `tail -f` the file.  Having the `_log` override part clutters up the README.md, and makes the PHP SimpleExperiment less consistent with the documentation on the github.io page.