Me remembers rewan-something, sorry i forgot ur name, but he wrote a tutorial on it. But here is an explanation.
Php is a language used to make dynamic web pages. What you have to do, is install php, from php.net (from apache) and configure your server to parse .php files through php.exe Php comes with preconfigured settings for most common server types. Now let me explain this parse thing, just some basic code, I wanna make a varible, so lets say I type
$khakis = "khakis r kewl";
Then that means that whenever I type $khakis in a php script on that page it will be replaced by khakis r kewl. Taking a closer look, (By the way, all php pages must end in .php or .php3 or .phtml) what happened was the server, seeing that the page was .php ran it through php.exe which saw
$khakis = "khakis r kewl";
and from then on replaced all $khakis with khakis r kewl, it finished running the php page through php.exe, then gave the server back just html code, and the server then gave it to the user. So we could have php use mysql as well, that way we could display things like databases, by doing something like,
$db = (localhost, root, dbnamehere)
and use a built in function in php, which is called mysql_connect(), that way the page would connect to mysql, and then we could pull out databases. Php is dynamic, open source, and free.
Quick review,
1. User tries to access .php page
2. Server seeing it is .php gives it to php.exe
3. Php.exe processes all the php scripts/code in the page and hands it back over to the server in plain old html
4. Server gives page to user
For a database it would be likem
1. User tries to access page
2. Server seeing it is .php, give it to php.exe
3. Php.exe sees that some info for the page is located in a database, and connects to it (a coder must type in the functions that allow php to connect to a database) and pulls out the information
4. Php.exe processes all the code, adds in the database info and hands everything over to the server
5. Server gives it to the user
Php has a great relationship with mysql, they work great together, with many built-in mysql functions, as well as apache functions. (of course! they make the thing!)
There are tons of books and sites that explain php much better than this. I just wanted to tell you php rules, it is a great tool, though I started using asp, , I have to, I get payed for it. But if I had the choice, I would take php.

EDIT: php also has built-in support for generating images, (cool, huh?) as well as for dynamically creating .pdf files (adobe acrobat documents).