Haml: Markup Haiku
Haml takes your gross, ugly templates and replaces them with veritable Haiku. Haml is based on one primary principal: [i]markup should be beautiful[/i]. Check out [url]http://haml.hamptoncatlin.com/[/url].
This is a guide to using Haml in CakePHP instead of the normal .ctp/.thtml files. Note: this uses phphaml which is for PHP5.
First of all, download the HamlParser and HamlView from http://cakeforge.org/snippet/detail.php?type=package&id=35. Place HamlParser.class.php in the app/vendors/haml/ directory and place haml.php in the app/views/ directory. Create the directory app/tmp/haml and make it world-writable (chmod 0777). This is where the compiled php files are placed. Next, in your AppController, put var $view = 'Haml';. Now all view files with the extension '.haml' will be parsed as Haml. That's it!
Here's a sample Haml layout (app/views/layouts/default.haml):
And here's a sample view (app/views/hello/index.haml):
As you can see, using helpers inside Haml is no trouble at all.
First of all, download the HamlParser and HamlView from http://cakeforge.org/snippet/detail.php?type=package&id=35. Place HamlParser.class.php in the app/vendors/haml/ directory and place haml.php in the app/views/ directory. Create the directory app/tmp/haml and make it world-writable (chmod 0777). This is where the compiled php files are placed. Next, in your AppController, put var $view = 'Haml';. Now all view files with the extension '.haml' will be parsed as Haml. That's it!
Here's a sample Haml layout (app/views/layouts/default.haml):
!!! Strict
%html
%head
%meta{ :http-equiv => 'Content-Type', :content => 'text/html;charset=utf-8' }
%title= $title_for_layout
%body
#header
%h1 hello
#content= $content_for_layout
#footer
%span.author John Q. Caker
And here's a sample view (app/views/hello/index.haml):
.hello
%p Hello, world!
= $html->link('This is a link to the home page', '/')
As you can see, using helpers inside Haml is no trouble at all.

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- ASP.NET MVC Framework (NHAML)
- Ruby (origin)
- Multiple implementations for PHP (PHPHAML on SF (as above) & pHAML)
If you're interested in the 'viability', or rather, how it can help you, check out the HAML entry on Wikipedia, that will help answer your questions. And, as always, TMTOWTDI.