XML based template engine versus Smarty lexer

There was a lot of excitement about Smarty 3 and its new lexer and how much more power it would give you as a template designer, but when it actually hit the shelves as it were, it was a real disappointment how slow it was. Compil开发者_高级运维ing a template from scratch took well over a second in Smarty 3, whereas the same template in Smarty 2 would take about half a second. Not good.

But it did get me thinking, why do you need to implement a full-blown language parser in PHP when it already has modules like DOMDocument, SimpleXML and the like available to it?

Are there any template engines for PHP that are based on the XML extensions and/or DOMDocument? If so, what is the performance like? If not, then has anybody attempted to write one?

One drawback I can forsee is it would only really be useful for XML-based formats such as XHTML and RSS. For generating other outputs (Non-XML HTML, plain text, CSS, etc) it would potentially be quite problematic, though I'm sure you could get round it with CDATA blocks. Are there any other implications for using XML/DOM for template parsing that I've not considered?

On your point about Smarty, IIRC Smarty uses "compiled templates", so if the performance issue you mention is only in the "compile phase" is becomes a moot point - each template only compiled once, thereafter the template content is output from (much faster) cache.

The problem with using an XML parser is that HTML isn't always well formed XML. Even if you use valid XHTML you can end up jumping through hoops to support HTML entities, and then you'll find corner cases like embedded Javascript, etc. (On a side note, IMO this is HTML5's biggest failure - it doesn't deprecate all that legacy SGML crap and insist on using well-formed XML. If the HTML committee had done this then future templating engines would be much easier to write using standard XML APIs.) I wrote an XML-based template engine some time ago, this uses the XMLReader API, but to make it work with HTML you have to add entries in to your Libxml system catalog. This works well enough but is a pain, most people would just give up and use something simpler.

Here are some of the XML-based template engines I know of:

http://phptal.org/

http://code.google.com/p/querytemplates/

http://www.hyperkit-software.com/projects/phptemplates/index.html

In terms of performance, I don't think most XML-based template engines are significantly faster in the compile phase - most modern template engines for PHP use compilation, so performance of the compiler is generally sacrificed in favor of a more extensible and maintainable engine codebase, as well as producing more optimized compiled templates. As Robin pointed out, because the templates are compiled, no one really cares how fast or slow the compilation phase may be.

There are other arguments for XML-based template engines, however. Personally, I dislike the idea of mixing two different tag-syntaxes, one of which (Smarty) cannot be parsed, processed or validated using ordinary XML-tools. To clarify, yes, you can parse the XML portion of a Smarty template, but you cannot parse or modify the Smarty-tags, which look like text to an XML tool.

Another common argument against custom syntax (such as used in Smarty and most other template engines), is that PHP already has syntax for everything these template engines provide - for example, <?=ucfirst($person->name)?> is similar to {$person.name|ucfirst} in a plain PHP template. It uses a syntax that is already known to PHP developers, which means there is no learning curve, no compilation step, no engine that needs to be deployed, no runtime overhead to render the template, etc.