What is hook_theme()?

hook_theme() serves as a fundamental component in Drupal 7's theming system. It provides a centralized location where modules can define their theme implementations, which can then be customized by other modules or themes. This flexibility allows developers to modify how variables are processed before they reach the final theme function or template file.

Implementing Theme Functions

The basic implementation of a theme function follows this pattern:

theme('poll_results', array(
   'raw_title' => 'title',
   'results' => $results,
   // Additional variables as needed
));

Understanding Render Arrays

In Drupal 7, render arrays provide a structured way to build output. Here's how to create one:

$build = array(
   '#theme' => 'poll_choices',
   '#form' => $form
);

The '#theme' key tells Drupal which theme implementation to use, while other keys provide the data needed for rendering. This approach offers better flexibility and maintainability compared to direct HTML output.

## Defining Theme Implementations

When defining a theme implementation that uses a form, you would structure it like this:

$hooks['poll_vote'] = array(
   'template' => 'poll-vote',
   'render element' => 'form',
);

Let's break this down:
- 'template': Specifies the template file name (without the .tpl.php extension)
- 'render element': Indicates which variable contains the primary data structure (in this case, a form)

Theme Functions vs Template Files

You have two options for implementing theme output:

1. Theme Functions:

function theme_poll_results($variables) {
   $raw_title = $variables['raw_title'];
   $results = $variables['results'];
   // Process and return themed output
   return $output;
}

2. Template Files (poll-results.tpl.php):

<?php
// Variables are automatically extracted from $variables
// So $raw_title and $results are directly available
?>
<div class="poll-results">
   <h3><?php print $raw_title; ?></h3>
   <!-- Additional themed output -->
</div>

When to Use Each Approach:

Template Files are preferred when:
- You have substantial HTML output
- You want to separate markup from logic
- The output structure needs to be easily modified by theme developers

Theme Functions are better when:
- The output is relatively simple
- You need complex logic to generate the output
- The output format varies significantly based on conditions

Best Practices

1. Always document your variables:

/**
* Implements hook_theme().
*/
function mymodule_theme() {
   return array(
       'poll_results' => array(
           'variables' => array(
               'raw_title' => NULL,
               'results' => NULL,
           ),
       ),
   );
}

2. Use meaningful variable names that describe their purpose
3. Provide default values when appropriate
4. Consider preprocess functions for complex logic
5. Keep template files focused on presentation logic only

This structured approach to theming ensures your code remains maintainable and allows for maximum flexibility in customizing output. Remember that good theming practices contribute significantly to the long-term sustainability of your Drupal project.