How many explode functions per page do you think is ok?
At the moment i have 3 explode function which splits the page up, gets and displays a title / chapter and one to echo the title of the next page.
How much would this slow my pages down?
Probably not more than running two queries on your page. I normally don't worry about stuff like that until it poses a problem if I start to see a slow down in performance then I look for the cause, but I am pretty sure there is no standard that dictates how many of each php function you should have on a page. From my experience I try to keep it to one query, and not a lot of ereg().
I only have one query but three explodes that is all...
So i should be OK...