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
Nope, not using this in any way. In our implementation it's currently the best way of signaling how to use an operator but would probably make more sense just being incorporated into the operator comments instead of being its own function...