I explained to you quickly via aim. PHP is a server side language that outputs HTML. In lamens terms - the server will read the php code and output (usually dynamic) html code that browsers can read. When you make a post, it sends data (using a php script), to a database. The code then reads off the database (MySQL), and outputs your post. PHP is quickly becoming the most popular web scripting language - mainly because its very user friendly and available on most servers.
Hope this helps out a bit more than I could provide earlier. If you have any scripting questions - you could always chat with my programming-guru brother, demo.
