[Drupal] Programtically add custom meta tags to pages.

Assuming that we need to add custom meta tags by page url or node type, drupal hook_preprocess_page() is the last function called before displaying the page, so it's the right place for doing that :

/**
 * Implementation of hook preprocess_page
 *
 * @param array $variables
 */
function my_module_preprocess_page(&$variables) {
    //using path
    if(arg(1) == 'gallery'){
      //load existing node
      $node = node_load(arg(2));
     // the meta content.
      $head = ' .. ';
      drupal_set_html_head($head);     
      $variables['head'] = drupal_get_html_head();
  }
}

In this case path is : gallery/[nid]