PHP is pretty much a templating system. The key is to force yourself to separate logic from presentation on your own. Using Smarty or something like that only makes it slightly more inconvenient to mix logic and presentation. If you can't make yourself separate them on your own, using a templating system isn't going to help. All it's going to do is eat up additional processing power.
The key is to not alter any values in your presentation code. To do this, I think PHP itself is just as effective as Smarty if you use the if/endif syntax:
Some text!
