Writing an own syntax highlighter
A syntax highlighter for PhD is nothing more than a simple PHP class that has two methods, a factory and highlight.
factory is static and takes the format name (i.e. pdf, xhtml, troff) as only parameter. It returns the highlighter instance object for the given format. The method is called for each output format the documentation is rendered to.
highlight takes three parameters, text, role and format. It is called whenever a piece of source code needs to be highlighted and expects the highlighted source code to be returned in whatever format the current rendering format is expected to be.
Take a look at the provided highlighters, phpdotnet\phd\Highlighter, phpdotnet\phd\Highlighter_GeSHi and phpdotnet\phd\Highlighter_GeSHi11x. They will serve as good examples how to implement your own highlighter.
Once you wrote your custom source code highlighting class, it's time to try it out.