How to add comments via PHP?

Can anyone suggest a way to add comments to an entry via PHP?

Reason:

I want to build a booking system, with each inventory item as a channel entry, authored by the borrower. When an item is overdue, I want to add a comment to the entry (via PHP), with channel notification preferences set to email the author when a new comment is added.

There’s probably other ways to build a system like this, but I think using the commenting system might be a pretty nifty way, since the thread of comments become sort of a rolling tally of notifications relating to that item.

I just don’t know how to actually add comments to an entry via PHP – can anyone suggest a method?

Thanks!

[Mod Edit: Moved to the Development and Programming forum]