PHP is not a template engine, but a language that can be used to write templates, or template engines. A template engine is not just a language, but also the programming API that allows the scripts to locate, organize templates or assign the data from the script to them. Pure PHP offers you absolutely nothing - it is just a language. Instead, you should take such libraries, as Zend_View in Zend Framework to comparisons (basically, it works exactly in the same way, as Smarty, except that it uses PHP to write templates). You should ask whether you should use a template engine with PHP or something else as a template language.
When it comes to templating languages themselves, then well... ordinary loops and conditions are enough to write templates, but this "enough" does not mean that it is easy, comfortable, efficient or flexible. PHP does not offer anything special for template designers, but many "templating languages" (like Smarty) provide just a limited subset of PHP, so I'm not surprised that programmers choose PHP. At least they can write functions and use OOP which is too massive for this (in my opinion), but does work and really helps.
The point is that custom templating languages are not limited with PHP drawbacks, but their designers unsually do not see it, claiming that "displaying variables and a loop is enough". The possible areas, where templating languages could be much more effective:
- Form displaying and rendering (I haven't seen any framework with PHP as a templating language that provided an easy, flexible and generic system for customizing the form look).
- Understanding the HTML/XML document structure.
- Automatic XSS injection filters.
- Solving various common problems in the presentation layer (i.e. customizing the look of the pagination system, displaying the data in columns etc.)
- Template portability and true separation of the application logic and implementation details from the templates.
Examples of templating languages that follow this way are mentioned above PHPTAL and Open Power Template 2. Some similar ideas can be also found in TinyButStrong, but unfortunately this template engine is extremely slow.
