Drupal 7 Module Development

Drupal’s Theme Layer
Now, if you were paying close attention to the form of the name, you'll also notice
that these functions actually receive two parameters, namely, the $variables array
and a $hook parameter. $hook, as the name suggests, contains the name of the
actual theme hook currently being run. So, while a foo_preprocess(&$variables,
$hook) function is run for every template file, it will still be able to tell which
template is currently being requested. In fact, $hook is the second parameter for all
preprocess functions, but $hook is only useful for multi-hook preprocess functions.
For a good example of a multi-hook preprocess function, let's look at the function
that the theme system uses to set up several variables common to all template
files—the template_preprocess() function, which is as follows:
function template_preprocess(&$variables, $hook) {
// Tell all templates where they are located.
$variables['directory'] = path_to_theme();
}
// Initialize html class attribute for the current hook.
$variables['classes_array'] = array(drupal_html_class($hook));
As you can see, this preprocess function creates a $directory variable which
can be used to tell where the template file is located on the web server. In the
$classes_array variable, it also starts to set up the CSS classes used in the
outer-most wrapping div of the template.
Process functions
Obviously, inside our template file, when we print out our dynamically created list
of classes, we'll need the variable to be a string.
will, most unhelpfully print out "array". In earlier versions of Drupal, classes were
dynamically created but were immediately concatenated into strings. So themes would
see one long string with multiple classes in it, menu-block-wrapper menu-block-1
menu-name-management, for example. This made removing or altering classes difficult
as themers had to master PHP's string-manipulation functions or even (gasp!)
regular expressions.
In Drupal 7, this problem for themers has been solved using the new process
functions. Process functions are an additional phase of variable processing
functions that run after the initial preprocess functions. In all respects, process
functions are exactly like preprocess functions; there are template_ prefixed process
functions, multi-hook process functions, module-provided process functions, and
theme-provided process functions. The only difference is that process functions are
run after all preprocess functions have been run.
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