Yes, I've built a couple of large PHP applications handing gnarly business logic that did not turn into spaghetti code. The been easy enough to maintain and extend over the years as well. OOP is used for the persistence code only.

The trick to this has been to have minimal code in the MVC layer, and keep most of the logic in functions which are organized in into per topic files.

So when my application wants to find out the price of something, I'll call price($product, $user) from pricing.php, which will then handle all the voodoo business rules about pricing (does the user get a member discount? what kind of discount do they get? do they own a related product? is it Friday the 13th?) price() depends on other functions like has_active_membership() defined in the file about subscriptions, and on membership_discount_pct() in the pricing file.

This function oriented breakdown applies not just to business logic, but authentication, charging credit cards through stripe, sending emails, course status, editing rights, etc.

I deliberately do not use any hooks or other meta chicanery. It's plain functions calling plain functions all the way down.