A short overview of the files:

Somewhat obvious... $_GET values:

id → the board id
action → what action file should be loaded in the bottom section (see below)
- valid actions: [not present], post, submit, edit
end → if present, jump to the end of the page

the top part of a board page, displays the posts, loaded via include()

one of the action files for the bottom of the page, displays a form to add a post, loaded via include()

displays a message in the bottom of the page and submits a new post, loaded via include()

? [Will look into it later, seems not to do very much ATM]

displays a button giving the user the option to post something

this file