I'm afraid I don't understand your question. Can you perhaps rephrase?
If you are looking for how to use the codes on the board, just put the code (B,I,U,URL,IMG, etc.) between square brackets ([]), and then end it using a [/] bracket, also containing the code.
Example: [php]this is PHP code[/php]
If you want to know how to use codes in your own forum, then you'll have to use regular expressions, like this:
$code = eregi_replace("\\[url=\"?([^\\[]*)[\"?]\\]([^\\[]*)\\[/url\\]","<a href=\"\\1\" target=\"_blank\">\\2</a>",$code);
This works exactly like the vBulletin [url] tag.