I am relatively new to PHP, but experienced Java programmer in complex enterprise environments with SOA architecture and multitier applications. There, we'd normally implement business applications with business logic on the middle tier.
I am programming an alternative currency system, which should be easy deployable and customizable by individuals and communities; it will be open source. That's why php/mysql seems the best choice for me.
Users have accounts, and they get a balance. also, the system calculates prices depending on total services delivered and total available assets.
This means, on a purchase a series of calculations happen; the balance and the totals get updated; these are derived figures, something normally not put into a database.
Nevertheless, I resorted to putting triggers and stored procedures into the db, so that in the php code none of these updates are made.
What do people think? Is that a good approach? My experience suggests to me that this is not the best solution, and prompts me to implement a middle tier. However, I would not even know how to do that. On the other hand, what I have so far with store procs seems to me the most appropriate.
I hope I made my question clear. All comments appreciated. There might not be a "perfect" solution.
As is the tendency these days, getting away from the DB is generally a good thing. You get easier version control and you get to work in just one language. More than that, I feel that stored procedures are a hard way to go. On the other hand, if you like that stuff and you feel comfortable with SPs in MySql, they're not bad, but my feeling has always been that they're harder to debug and harder to handle.
On the triggers issue, I'm not sure whether that's necessary for your app. Since the events that trigger the calculations are invoked by the user, those things can happen in PHP, even if the user is redirected to a "waiting" page or another page in the meantime. Obviously, true triggers can only be done on the DB level, but you could use a daemon thread that runs a PHP script every X seconds... Avoid this at all costs and try to get the event to trigger from the user side.
All of this said, I wanted to plug my favorite solution for the data access layer on PHP: Doctrine. It's not perfect, but PHP being what it is, it's good enough. Does most of what you want, and keeps you working with objects instead of database procedures and so forth.
Regarding your title, multiple tiers are, in PHP, totally doable, but you have to do them and respect them. PHP code can call other PHP code, and it is now (5.2+) nicely OO and all that. Do make sure to ignore the fact that a lot of PHP code you'll see around is total crap and does not even use methods, let alone tiers, and decent OO modelling. It's all possible if you want to do it, including doing your own (or using an existing) MVC solution.