Why should I use templating system in PHP?

Using non-PHP templates with the excuse of separating logic is nonsense. If the developer doesn't understand what the business-view logic separation is and how it should be done, then the problem must be addressed appropriately. Otherwise you end up with HTML in business logic or business logic in templates -- no templating engine is going to save you. You have to teach the developer the basics.

And if the developer does understand that, the templating system is only a limitation. It doesn't add any value to the development process, only overhead of learning a new syntax, keeping another library up to date, and slower execution. While the latter can be solved with caching and whatnot, this only remedies a problem that otherwise wouldn't exist. So, templating systems offer no value, no advantage at all.

There is one exception, though, where I think using a non-PHP templating system is reasonable: when view-logic programmers must have limited access to the templates. For example if you're a provider for a blog-hosting system and you want to allow your users to personalize and code their templates, without allowing them to execute arbitrary code. This argument, however, does not apply to cases where a designer is willing to learn a little code to help programming the UI. If he can learn Smarty, he can surely learn PHP.

The main reason people use template systems is to separate logic from presentation. There are several benefits that come from that.

Firstly, you can hand off a template to a web designer who can move things around as they see fit, without them having to worry about keeping the flow of the code. They don't need to understand PHP, just to know to leave the special tags alone. They may have to learn a few simple semantics for a few tags but that is a lot simpler than learning the whole language.

Also, by splitting the page into separate files, both programmer and designer can work on the same 'page' at once, checking in to source control as they need, without conflicts. Designers can test their template visuals against a stable version of the code while the programmer is making other, potentially breaking changes, against their own copy. But if these people were both editing the same file and had to merge in different changes, you can encounter problems.

It also enforces good programming practice in keeping business logic away from presentation logic. If you put your business logic mixed in with the presentation then you have a more difficult time extracting it if you need to present it differently later. Different modes of presentation in web apps are increasingly popular these days: RSS/ATOM feeds, JSON or AJAX responses, WML for handheld devices, etc. With a template system these can often be done entirely with a template and no or little change to anything else.

Not everybody will need or appreciate these benefits however. PHP's advantage over Java/Python/Ruby/etc is that you can quickly hack up web pages with some logic in them, and that's all well and good.

Yes, as you said, if you don't force yourself to use a templating engine inside PHP ( the templating engine ) it becomes easy to slip and stop separating concerns.

However, the same people who have problems separating concerns end up generating HTML and feeding it to smarty, or executing PHP code in Smarty, so Smarty's hardly solving your concern separation problem.

See also:

• PHP as a template language or some other templating script
• What is the best way to insert HTML via PHP