I know I'm not 'supposed' to be posting in this forum, but I'd just like to point out that you could use a set of guidelines which are endorsed by other big open source projects such as phpBB, Joomla etc.

I personally prefer the phpBB3 • Coding Guidelines
. Not all of the things in that document would relate to your project, but most of it is good stuff.

You could alternatively follow the PEAR guidelines