Drupal 7 Module Development

Drupal’s Theme Layer
If a function were defined as follows:
function foo_spaghettify($code, $type = 'thin', $sticky = TRUE) { }
Then, code that called it could just call foo_spaghettify($code) and the $type and
$sticky parameters would get the default values defined in the function definition.
With a single $variables parameter we have an issue. If we tried the following:
function foo_spaghettify($variables = array('code' => array(),
'type' => 'thin', 'sticky' => TRUE) { }
And then called:
$variables = array('code' => $code);
$result = foo_spaghettify($variables);
Then, we would discover that $variables['type'] and $variables['sticky']
don't get the default value, and they are also undefined. The reason is simple; default
parameter values only get used if the parameter is not specified when calling the
function, but we did specify the $variables parameter, so its default is not used.
So how do we solve this problem in Drupal? We have modules that define the
default variables in their hook_theme functions.
hook_theme
A module's hook_theme is primarily responsible for specifying a few things,
as follows:
• The theme hooks that the module is responsible for
• The type of theme implementation (theme function or template)
• The theme hooks' default variable values
• If the hook expects a render element instead of variables
In addition to those main responsibilities, the hook_theme can optionally specify:
• Which file contains the theme function or preprocess functions (if it isn't in
the main module file)
• A pattern to use during the auto-discovery search of a theme's overridden
theme hook suggestions
• Some other esoteric things you can read about in its documentation:
http://api.drupal.org/api/function/hook_theme/7
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