PHP Markdown Extra by Michel Fortin was a WordPress Plugin that provided markdown support for WordPress. It was based on John Gruber's standard.
The plugin has since been long abandoned

As of March 2023, the Ministry of Justice reformatted the way the plugin is loaded and maintained. For instance, the latest versions of the classes provided by Michel Fortin are imported, by hand, into the class/ directory. Using composer, these classes are further auto-loaded, and initialised using class/Bootstrap.php

The following process for maintaining the plugin presents a simple method.
Improvement is encouraged.

1. Visit the lib directory in GitHub: Michel Fortin
2. Check the version data against our local files, update if needed
3. Copy the latest code for:
   1. Markdown.php
   2. MarkdownExtra.php, and
   3. MarkdownInterface.php
4. Replace local code and commit.

Further information can be found at https://michelf.ca/projects/php-markdown/extra/