It's fairly typical in phpBB forums that edits show if you change something after the next person posts. Most people edit for spelling or grammar, but if there's any one person you're concerned about you can always "quote" them.

Unless it's an organized debate thread, probably few would care.

A post can be edited many times over, over any amount of time.
There is a notification at the bottom-most area of any post that has been edited, along with the date (of the latest edit) and how many occasions it has been edited.
However, this only occurs if someone has replied.

So if you were to edit your above post five times before i replied - it would not show that you edited it. This is because often (at least this is the case for me) a post is made, and then as you re-read your own post you spot an error or two that require editing. As long as no-one has replied to your post, it can be edited without any notification. This way, simple errors (grammatical, spelling, etc.) can be modified in a relatively short time.
However, let's say an argument is occuring between two members, and member-one said in post#123 "well, i hate your guts because you smell". Member-two then states "I do smell, yes! Lovely! I smellllll gorgeous!!", in response - member-one might edit their post to say "....you smell BAD!". That would then leave a notification for others that an edit was made.

OK, it's a crap example - but it's all I could muster for the moment.

ETA: I've just done that now.. re-read my own post, and spotted errors LOL. For clarity, on this occasion, i'll highlight my own editing.

Ah, gotcha.

Yeah, I was just correcting spelling mistakes and wondered why the edit wasn't shown.

I've just seen a load more but I won't bother to correct them lest somebody suspects shenanigans.

I suspect shenanigans! I do, I tell ya, I do!

Quote from Karl

I suspect shenanigans! I do, I tell ya, I do!

Quote everyone, every post!