Hopefully you are starting to see the potential of server-side scripting.  It can also be abused if you don't do it right.  So here are some warnings and then some practise to make sure you know what you are doing.  As ever this is just a start and if you are not making sites for real people you should start soon.  You should also be reading books, articles, forums and other sites to make sure any gaps in your knowledge are covered.