I want to create my own custom PHP,MySQL forum

Surely if you are creating your own then you will write the source code your self...

Or do you mean you want an open source one?
Try:
Vanilla
or
phpBB

Yes I want to create my own. But i just want sample code so that I develop mine from there!

you need decide the navigation of your forum site, then the code need to develop that forum will available on the internet individually...you just start it,if any codes requires ,post here,many daniweb code gurus will help you...And mind one thing , develop it by SEO friendly..And follow some security instuctions...
All the best...

Shanti.

A forum differs slightly from a custom CMS or a social networking site. The threads and posts can be viewed like the news on a backend. The most important points to get focus are: 1) the ACL or authentication part taking into account the moderators and users, as well as registration and ban process. 2) URL Rewriting, convert thread titles to SEO ones. 3) A parser to interpret BBCode tags on the input. Also the input window can be ornamented with Javascript or using a WYSIWIG editor like FCKeditor or TinyMCE with disabled features to make it very similar to vBulletin editor showed on this site.

For learning purposes (I like pretty much phpBB but seems very entangled) I suggest to start with SMF forum from simplemachines, it's free and open source too. The code, in special about polling features are very clear.