There are options other than 'Pay-per-click' (which, IMHO, just plain old sucks, and would drive me outta here faster than anything else) and 'subscription' services.
With all due respect to burgenets post that banner ads tend to generate marginal revenue (at best), and yes, someone has gotta foot the bills for hosting and administrative costs, perhaps other options could be explored...?
Perhaps a sponsorship agreement from corporate sources which have benefitted from PHP?
Perhaps a quid-pro-quo agreement with businesses, wherein administrative and hosting costs are exchanged for PHP-based programming services?
Perhaps donated hosting and administrative services from educational institutions? I could definitely see a forum such as this, as well as its administrative and maintenance issues, fit appropriately into a computer science curriculum.
I'd be willing to bet that subscription or pay-per-click options would drive away far too many people, especially when there seem to be so many folks participating herein that provide more 'solutions' than 'issues'. If I found out that some fat 'suit' was driving home in Ferrari that was paid for by folks who freely contributed their time and talents... well, I'd take that as a real dis-incentive to offer any more free support.
All this leads to the crux of the biscuit of open source software in general: somebody's gotta pay the bills for the administrators, bandwidth providers and the hardware while the programmers give away the code. The typical solution attempted (RedHat and MySQL AB as examples) is 'give away the product, sell the support.' Which, sadly, is successfully acheived by very few companies.
In a forum such as this, the support can be somewhat of a crapshoot. If I'm going to pay money for solutions, then, dammit, I want definitive answers from competent support personnel and not stuff that starts with, "You might wanna try..."
Oddly enough, one of the reasons that I started using open-source software exclusively is because I find the actual 'support' to be better than any of the 'customer-service' experiences that I've had with Micro$oft, IBM or Sun, and there have been a few, I assure you.