Keith Casey has a great series of points about joining a startup here, especially the part about founders who "believe in themselves".

To this, I must also add a recommendation to read Gerber's The E-Myth for entrepreneurial-minded programmers thinking about starting a business of their own.

Are you stuck with a legacy PHP application? You should buy my book because it gives you a step-by-step guide to improving you codebase, all while keeping it running the whole time.