I cant remember the name of this movie. This is an old movie probably late 90s.  I cant confirm all of the following, but this is how I remember it..

An american guy gets caught in this mystical world and has to learn martial arts to defeat a bad guy or something. He gets a teacher, I guess jet li or jackie chan teaches him kung fu.

One specific detail I remember is, our hero is given a writing brush by an old guy as a weapon. That little brush can transform into a huge staff for fighting..

At some point, our hero has a fight with his teacher (jet li or jackie chan or whoever) and hits him with the magic brush/staff thingy which splits the teachers head in two. But being a mystical guy living in a magical world , he is not hurt.. there is bright light coming from the spilt head and he just kinda pulls himself together and says “is this how you treat your master?”

I cant seem to remember any other detail apart from this. Any kind of lead will be helpful for me as I cant sleep properly from now on without knowing what movie I remembered.

Thanks.