Pierre has a quick post on his blog about the highlighting of PHP code in his blog entries and mentions some of the tools that can add it to your scripts.

Scripts and codes in my posts always look sad, gray. I knew there a syntax highliter plugin for dotclear. It works very well and is relatively fast (being in php...). This plugin uses GeSHi (Generic Syntax Highlighter), a porwerfull and easy library.

The other tools that he lists are: PeclColorer, PEAR TEXT_Highlighter, and Beautifier. Of course, there's always the PHP function highlight_string, but it's also not very full-featured. There are also plugins for other blogs as well, such as PHP Highlighter for Wordpress.