there are a couple of ways to make a auto posting bot in php
curl/php is probably the easy way
i have just been working on a similar thing for my phpbb3 forum, i have made the curl script in php to 1st, extract a post form another forum
2nd log into my forum and naviagte to a perset forum section
3rd post the copyed info into my forum
problem is tho you need to extract first all the urls of the target forum into a big list, then run the bot threw the list copying and posting threads, i use it to target specfic forum sections one at a time, mostly porn sharing forums
thinking of maybe automating it a bit futher with windows macros to run threw one list then move onto another list, could also be used to spam all memebrs n the members list to get them over to your site also
let me no if anyone is interseted in this , and i will post up the code, so we can futher make it into a complete vbuiliten posting bot