Smarty is a template engine for PHP. More specifically, it facilitates a manageable way to separate application logic and content from its presentation. This is best described in a situation where the application programmer and the template designer play different roles, or in most cases are not the same person.
For example, let's say you are creating a web page that is displaying a newspaper article. The article headline, tagline, author and body are content elements, they contain no information about how they will be presented. They are passed into Smarty by the application, then the template designer edits the templates and uses a combination of HTML tags and template tags to format the presentation of these elements (HTML tables, background colors, font sizes, style sheets, etc.) One day the programmer needs to change the way the article content is retrieved (a change in application logic.) This change does not affect the template designer, the content will still arrive in the template exactly the same. Likewise, if the template designer wants to completely redesign the templates, this requires no changes to the application logic. Therefore, the programmer can make changes to the application logic without the need to restructure templates, and the template designer can make changes to templates without breaking application logic.
Now for a short word on what Smarty does NOT do. Smarty does not attempt to completely separate logic from the templates. There is no problem with logic in your templates under the condition that this logic is strictly for presentation. A word of advice: keep application logic out of the templates, and presentation logic out of the application. This will most definately keep things manageable and scalable for the foreseeable future.
One of the unique aspects about Smarty is the template compling. This means Smarty reads the template files and creates PHP scripts from them. Once they are created, they are executed from then on. Therefore there is no costly template file parsing for each request, and each template can take full advantage of PHP compiler cache solutions such as Zend Accelerator (http://www.zend.com) or PHP Accelerator (http://www.php-accelerator.co.uk).
Some of Smarty's features:
* It is extremely fast.
* It is efficient since the PHP parser does the dirty work.
* No template parsing overhead, only compiles once.
* It is smart about recompiling only the template files that have changed.
* You can make custom functions and custom variable modifiers, so the template language is extremely extensible.
* Configurable template delimiter tag syntax, so you can use {}, {{}}, <!--{}-->, etc.
* The if/elseif/else/endif constructs are passed to the PHP parser, so the {if ...} expression syntax can be as simple or as complex as you like.
* Unlimited nesting of sections, ifs, etc. allowed.
* It is possible to embed PHP code right in your template files, although this may not be needed (nor recommended) since the engine is so customizable.
* Built-in caching support
* Arbitrary template sources
* Custom cache handling functions
* Plugin architecture
