Html is a necessity. But for business websites, so is PHP. You can design an entire site out of PHP although (it would take pretty long). It is a good idea to use both. PHP is secure, no one can see PHP script, so it is used mainly on sites with members, e-commerce, and stuff you don't want available to hackers.
Flash is also very cool, although too much flash can get annoying, as well as making it harder for search engines to document your pages. This is because search engines can't read the text and pictures that you put in the flash file. So it is good to use flash in moderation.