league/commonmark is the standard Markdown library for PHP: CommonMark- and GFM-compliant, with an AST, an event system and many bundled extensions (tables, task lists, footnotes, front matter, heading permalinks, table of contents, attributes, embeds, mentions). Laravel and Drupal use it. Pick it for new code.
Parsedown is a single-file parser that’s fast and simple. It doesn’t follow CommonMark and supports only part of GFM (tables, fenced code, strikethrough, autolinks); ParsedownExtra adds footnotes, definition lists and attributes. It’s in many older projects.
league/commonmarkleague/commonmark is the CommonMark and GitHub Flavored Markdown parser for PHP. How to convert Markdown to HTML safely, enable GFM and extensions.
ParsedownParsedown is a fast, single-file Markdown parser for PHP. How to install it, convert Markdown to HTML, enable safe mode for user input and use ParsedownExtra.