How to create effective PHP project documentation with Read the Docs

Documentation is one of the ways that software projects can communicate information to their users. Effective documentation is high-quality, meaning that it’s complete, accurate, and up-to-date. At least for open source libraries, it also means that you can find it with a search engine. For many small PHP projects, the reality is very far removed from the ideal.

Read the Docs (readthedocs.io) makes it easy to host an up-to-date copy of your project’s documentation online. There are around 2,000 PHP projects which host their documentation on the site, which makes PHP the third most popular programming language for projects on the site.

This post covers the process that is used to automatically publish the documentation for the gfx-php PHP graphics library, which is one of those 2000 projects. You should consider using this setup as a template if you your project is small enough that it does not have its own infrastructure.

Basic concept

Typically, people are using Read the Docs with a tool called Sphinx. If you are writing in Python, it’s also possible to use the autodoc Sphinx plugin to add API documentation, based on docstrings in the code.

PHP programmers are already spoiled for choice if they want to produce HTML documentation from their code. These tools all have huge PHP user bases:

These will each output their own HTML, which is only useful if you want to self-host the documentation. I wanted a tool that was more like “autodoc for PHP”, so that I can have my API docs magically appear in Sphinx output that is hosted on Read the Docs.

Doxygen is the most useful tool for this purpose, because it has a stable XML output format and good-enough PHP support. I decided to write a tool which to take the Doxygen XML info and generate rest for Sphinx:

This introduces some extra tools, which looks complex at first. The stack is geared towards running within the Read the Docs build environment, so most developers can treat it as a black box after the initial setup:

This setup is entirely hosted with free cloud services, so you don’t need to run any applications on your own hardware.

Tools to install on local workstation

First, we will set up each of these tools locally, so that we know everything is working before we upload it.

  • Doxygen
  • Sphinx
  • doxyphp2sphinx

Doxygen

Doxygen can read PHP files to extract class names, documentation and method signatures. Linux and Mac install this from most package managers (apt-get, dnf or brew) under the name doxygen, while Windows users need to chase down binaries.

In your repo, make a sub-folder called docs/, and create a Doxyfile with some defaults:

mkdir docs/
doxygen -g Doxyfile

You need to edit the configuration to make it suitable or generating XML output for your PHP project. The version of Doxygen used here is 1.8.13, where you only need to change a few values to set Doxygen to:

  • Recursively search PHP files in your project’s source folder
  • Generate XML and HTML only
  • Log warnings to a file

For a typical project, these settings are:

PROJECT_NAME           = "Example Project"
INPUT                  = ../src
WARN_LOGFILE           = warnings.log
RECURSIVE              = YES
USE_MDFILE_AS_MAINPAGE = ../README.md
GENERATE_LATEX         = NO
GENERATE_XML           = YES

Once you set these in Doxyfile, you can run Doxygen to generate HTML and XML output.

$ doxygen

Doxygen will pick up most method signatures automatically, and you can add to them via docblocks, which work along the same lines as docstrings in Python. Read Doxygen: Documenting the Code to learn the syntax if you have not used a documentation generator in a curly-bracket language before.

The Doxygen HTML will never be published, but you might need to read see how well Doxygen understands your code.

The XML output is much more useful for our purposes. It contains the same information, and we will read it to generate pages of documentation for Sphinx to render.

Sphinx

Sphinx is the tool that we will use to render the final HTML output. If you haven’t used it before, then see the official Getting Started page.

We are using Sphinx because it can be executed by online services like Read the Docs. It uses the reStructuredText format, which is a whole lot more complex than Markdown, but supports cross-references. I’ll only describe these steps briefly, because there are existing how-to guides on making Sphinx work for manually-written PHP documentation elsewhere on the Internet, such as:

Still in the docs folder with your Doxyfile, create and render an empty Sphinx project.

pip install sphinx
sphinx-quickstart --quiet --project example_project --author example_bob
make html

The generated HTML will initially appear like this:

We need to customize this in a way that adds PHP support. The quickest way is to drop this text into requirements.txt:

Sphinx==1.7.4
sphinx-rtd-theme==0.3.0
sphinxcontrib-phpdomain==0.4.1
doxyphp2sphinx>=1.0.1

Then these two sections of config.py:

extensions = [
  "sphinxcontrib.phpdomain"
]
html_theme = 'sphinx_rtd_theme'

Add this to the end of config.py

# PHP Syntax
from sphinx.highlighting import lexers
from pygments.lexers.web import PhpLexer
lexers["php"] = PhpLexer(startinline=True, linenos=1)
lexers["php-annotations"] = PhpLexer(startinline=True, linenos=1)

# Set domain
primary_domain = "php"

And drop this contents in _templates/breadcrumbs.html (explanation)

{%- extends "sphinx_rtd_theme/breadcrumbs.html" %}

{% block breadcrumbs_aside %}
{% endblock %}

Then finally re-install dependencies and re-build:

pip install -r requirements.txt
make html

The HTML output under _build will now appear as:

This setup gives us three things:

  • The documentation looks the same as Read the Docs.
  • We can use PHP snippets and class documentation.
  • There are no ‘Edit’ links, which is important because some of the files will be generated in the next steps.

doxyphp2sphinx

The doxyphp2sphinx tool will generate .rst files from the Doxygen XML files. This was installed from your requirements.txt in the last step, but you can also install it standalone via pip:

pip install doxyphp2sphinx

The only thing you need to specify is the name of the namespace that you are documenting, using :: as a separator.

doxyphp2sphinx FooCorp::Example

This command will read the xml/ subdirectory, and will create api.rst. It will fill the api/ directory with documentation for each class in the \FooCorp\Example namespace.

To verify that this has worked, check your class structure:

$ tree ../src
../src
├── Dooverwhacky.php
└── Widget.php

You should have documentation for each of these:

$ tree xml/ -P 'class*'
xml/
├── classFooCorp_1_1Example_1_1Dooverwhacky.xml
└── classFooCorp_1_1Example_1_1Widget.xml

And if you have the correct namespace name, you will have .rst files for each as well:

$ tree api
api
├── dooverwhacky.rst
└── widget.rst

Now, add a reference to api.rst somewhere in index.rst:

.. toctree::
   :maxdepth: 2
   :caption: API Documentation

   Classes <api.rst>

And re-compile:

make html

You can now navigate to your classes in the HTML documentation.

The quality of the generated class documentation can be improved by adding docstrings. An example of the generated documentation for a method is:

Writing documentation

As you add pages to your documentation, you can include PHP snippets and reference the usage of your classes.

.. code-block:: php

   <?php
   echo "Hello World"

Lorem ipsum dolor sit :class:`Dooverwhacky`, foo bar baz :meth:`Widget::getFeatureCount`.

This will create syntax highlighting for your examples and inline links to the generated API docs.

Beyond this, you will need to learn some reStructuredText. I found this reference to be useful.

Local docs build

A full build has these dependencies:

apt-get install doxygen make python-pip
pip install -r docs/requirements.txt

And these steps:

cd docs/
doxygen 
doxyphp2sphinx FooCorp::Example
make html

Cloud tools

Next, we will take this local build, and run it on a cloud setup instead, using these services.

  • GitHub
  • Read the docs

GitHub

I will assume that you know how to use Git and GitHub, and that you are creating code that is intended for re-use.

Add these files to your .gitignore:

docs/_build/
docs/warnings.log
docs/xml/
docs/html/

Upload the remainder of your repository to GitHub. The gfx-php project is an example of a working project with all of the correct files included.

To have the initial two build steps execute on Read the Docs, add this to the end of docs/conf.py. Don’t forget to update the namespace to match the command you were running locally.

# Regenerate API docs via doxygen + doxyphp2sphinx
import subprocess, os
read_the_docs_build = os.environ.get('READTHEDOCS', None) == 'True'
if read_the_docs_build:
    subprocess.call(['doxygen', 'Doxyfile'])
    subprocess.call(['doxyphp2sphinx', 'FooCorp::Example'])

Read the Docs

After all this setup, Read the Docs should be able to build the project. Create the project on Read the Docs by using the Import from GitHub option. There are only two settings which need to be set:

The requirements file location must be docs/requirements.txt:

And the programming language should be PHP.

After this, you can go ahead and run a build.

As a last step, you will want to ensure that you have a Webhook set up on GitHub to trigger the builds automatically in future.

Conclusion

It is emerging as a best practice for small libraries to host their documentation with Read the Docs, but this is not yet common in the PHP world. Larger PHP projects tend to self-host documentation on the project website, but smaller projects will often have no hosted API documentation.

Once you write your docs, publishing them should be easy! Hopefully this provides a good example of what’s possible.

Acknowledgements

Credit where credit is due: The Breathe project fills this niche for C++ developers using Doxygen, and has been around for some time. Breathe is in the early stages of adding PHP support, but is not yet ready at the time of writing.

Quick statistics about documentation hosted on Read the Docs

Many open source projects host their online documentation with Read The Docs. Since I’m currently developing a tool for Read The Docs users, I recently took a look at the types of projects that are hosted there.

The data in this post is derived from the list of projects available in the public Read The Docs API, and has its limitations. In particular, spam, inactive, or abandoned projects are not filtered.

Totals

This data is current as of May 20, 2018. At that time, there were 90,129 total projects on readthedocs.io.

Starting out, I had assumed that the majority of projects had all four of these attributes:

  • Hosted in Git
  • Programmed in Python
  • Documented in English
  • Documentation generated with Sphinx

As it turned out, this particular combination is only used on 35.8% (32,218) of projects, so let’s take a look at how each of these vary.

Programming languages

The two main conclusions I drew by looking at the programming languages are:

  • Python is the largest developer community that is using the Read the Docs platform
  • A lot of projects are hosting documentation that is not tagged with a programming language
# % Projects Programming language
1 39.92% 35978 Not code (“just words”)
2 39.27% 35391 Python
3 9.27% 8354 (No language listed)
4 2.83% 2551 Javascript
5 2.29% 2060 PHP
6 1.15% 1040 C++
7 1.13% 1018 Java
8 0.85% 769 Other
9 0.77% 690 C#
10 0.71% 640 C
11 0.42% 380 CSS
12 0.34% 304 Go
13 0.31% 283 Ruby
14 0.31% 277 Julia
15 0.11% 96 Perl
16 0.06% 55 Objective C
17 0.06% 54 R
18 0.04% 36 TypeScript
19 0.03% 31 Scala
20 0.03% 30 Haskell
21 0.03% 29 Lua
22 0.02% 22 Swift
23 0.02% 15 CoffeeScript
24 0.02% 14 Visual Basic
25 0.01% 12 Groovy

Documentation languages

You might have guessed that English dominates in software documentation, but here is the data:

# % Projects Language
1 93.1% 83888 English (en)
2 2.4% 2178 Indonesian (id)
3 1.4% 1287 Chinese (zh, zh_TW, zh_CN)
4 0.5% 456 Russian (ru)
5 0.5% 425 Spanish (es)
6 0.5% 412 French (fr)
7 0.3% 254 Portuguese (pt)
8 0.3% 233 German (de)
9 0.2% 209 Italian (it)
10 0.2% 208 Japanese (jp)
0.6% 579 All other languages

In total, documentation on Read the Docs has been published in 54 languages. The “All other languages” item here represents 44 other languages.

Documentation build tool

The majority are using the Sphinx documentation generator. The table here counts sphinx, sphinx_htmldir and sphinx_singlehtml together.

# % Projects Language
1 92.5% 83364 Sphinx
2 6.9% 6229 MkDocs
3 0.6% 536 Auto-select

Version control

Git is the version control system used for the vast majority of projects.

# % Projects Version control
1 96.5% 86988 Git
2 2.1% 1879 Mercurial
3 0.9% 816 SVN
4 0.5% 446 Bazaar