Drupal 7 Module Development

Drupal’s Theme Layer
Theme engines
Some themes require alternate theme engines. Theme engines can provide alternate
template syntax, naming standards, and helper functions. Several theme engines are
available for download at http://drupal.org/project/theme+engines. However,
we won't be discussing any theme engines except for Drupal's default theme engine,
PHPTemplate. The PHPTemplate theme engine has been the default theme since
Drupal 4.7, has been continuously improved with each version, and has proven its
worth again and again. Over 99% of themes available for download on drupal.org
use the default PHPTemplate theme engine. All of the examples in this book assume
you are using PHPTemplate. So, enough said.
Two ways to theme
So now that we have a good understanding of higher level concepts, let's get down
to the nitty-gritty of theme implementations. As mentioned earlier in the chapter,
there are actually two different ways to implement a theme hook:
• Theme functions: pass data to a PHP function to wrap it in markup
• Templates: pass data to a template which is a PHP file mixed with markup
and PHP print statements
Let's look at each of these in turn.
Theme functions
For a module developer, the easiest type of implementation to understand is a theme
function. Theme functions just need to follow a few simple rules in order for them
to work properly.
First, the name of the theme function follows the pattern:
theme_[theme hook name]
Since the theme hook name is used directly in the theme function's
name, theme hook names have the same constraints on naming as
regular PHP function names; the only valid characters in theme hook
names are alphanumeric characters and underscores. So if a module
has created an example_format theme hook, it would implement it
with theme function named theme_example_format().
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