GMLP is the Goofy Markup Language Processor.
Markup Language because it converts input text formatted like Markdown and produces HTML output like Markdown.pl and it's variants.
Processor because the input format is not hard-coded but defined as PHP data.
And Goofy because the code is a bit bloated and confusing.
The PHP data – optionally supported by functions and/or closures – is specified on the command line:
./gmlp readme.md markdown > readme.htm
The PHP data to define "markup rules" is called a definition file. Included are files for Markdown, BBCode, HTML2MD, TXT2MD, CSS minify/unminify, strip comments and a few more as examples.
GMLP on Github. GMLP at SourceForge.
See also Simple Markup.