I just read 40 signs you really are a lousy PHP programmer. I know I've been guilty of many of these guidelines. Some of my favorites:
- return HTML, not data, strings, or objects
- use $_GET instead of $_POST for any destructive actions
- you've never heard of sql injection or cross-site scripting
- don't keep the different layers seperated using something like MVC
I think these are some of the biggest "gocha's" out there. Everyone developing web applications in any language should be aware of at least these guidelines, if not the rest of those on the list.