title: How to implement hook_theme in drupal 7?
body: <p>I created a new drupal 7 theme and trying to implement hook_theme at template.php like this:</p>
  
  <pre><code>function mytheme_theme($existing, $type, $theme, $path){
      return array(
          'mytheme_header'=&gt;array(
              'template'=&gt;'header',
              'path'=&gt;$path.'/templates',
              'type'=&gt;'theme',
          ),
      );
  }
  </code></pre>
  
  <p>then I placed header.tpl.php into templates directory and cleared all caches, and call theme function:</p>
  
  <pre><code>theme('mytheme_header', $vars);
  </code></pre>
  
  <p>and header.tpl.php likes this:</p>
  
  <pre><code>&lt;?php
  fb('calling header template');//the function of FirePHP to output debug info
  print '&lt;div&gt;Header&lt;/div&gt;';
  //...
  </code></pre>
  
  <p>I check Firebug and it get the info 'calling header template', it mean it had called header.tpl.php, but it didn't print the html code. What's wrong with my code?</p>