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Philosophy

What is Smarty?

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 variables with elements such as tables, div's, background colors, font sizes, style sheets, svg etc.
One day the programmer needs to change the way the article content is retrieved, ie 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 would require no change 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.

Goals

The Smarty design was largely driven by these goals:

clean separation of presentation from application code
PHP backend, Smarty template frontend
complement PHP, not replace it
fast development/deployment for programmers and designers
quick and easy to maintain
syntax easy to understand, no PHP knowledge necessary
flexibility for custom development
security: insulation from PHP
free, open source

Two camps of thought

When it comes to templating in PHP, there are basically two camps of thought. The first camp exclaims that "PHP is a template engine". This approach simply mixes PHP code with HTML. Although this approach is fastest from a pure script-execution point of view, many would argue that the PHP syntax is messy and complicated when mixed with tagged markup such as HTML.

The second camp exclaims that presentation should be void of all programming code, and instead use simple tags to indicate where application content is revealed. This approach is common with other template engines (even in other programming languages), and is also the approach that Smarty takes. The idea is to keep the templates focused squarely on presentation, void of application code, and with as little overhead as possible.

Why is separating PHP from templates important?

Two major benefits:

SYNTAX: Templates typically consist of semantic markup such as HTML. PHP syntax works well for application code, but quickly degenerates when mixed with HTML. Smarty's simple {tag} syntax is designed specifically to express presentation. Smarty focuses your templates on presentation and less on "code". This lends to quicker template deployment and easier maintenance. Smarty syntax requires no working knowledge of PHP, and is intuitive for programmers and non-programmers alike.
INSULATION: When PHP is mixed with templates, there are no restrictions on what type of logic can be injected into a template. Smarty insulates the templates from PHP, creating a controlled separation of presentation from business logic. Smarty also has security features that can further enforce restrictions on templates.

Web designers and PHP

A common question: "Web designers have to learn a syntax anyway, why not PHP?" Of course web designers can learn PHP, and they may already be familiar with it. The issue isn't their ability to learn PHP, it is about the consequences of mixing PHP with HTML. If designers use PHP, it is too easy to add code into templates that doesn't belong there (you just handed them a swiss-army knife when they just needed a knife.) You can teach them the rules of application design, but this is probably something they don't really need to learn (now they are developers!) The PHP manual is also an overwhelming pile of information to sift through. It is like handing the owner of a car the factory assembly manual when all they need is the owners manual. Smarty gives web designers exactly the tools they need, and gives developers fine-grained control over those tools. The simplicity of the tag-based syntax is also a huge welcome for designers, it helps them streamline the organization and management of templates.

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