It's probably best to do this in a virtualenv environment, so set one up first:

pip install virtualenvwrapper
mkvirtualenv formatphp-docs
cd docs/
workon formatphp-docs
pip install -r requirements.txt

To build the docs, change to the docs/ directory, and make sure you're working on the virtualenv environment created in the last step.

cd docs/
workon formatphp-docs
make html

Then, to view the docs after building them:

open build/html/index.html