new hook_theme

hooke_theme_functions.php

/**
 * Implements hook_theme().
 */
function cogito_theme($existing, $type, $theme, $path) {
  global $theme;
  $path_to_child = "/" . drupal_get_path('theme', $theme) . '/';
  $path = is_file( getcwd() . $path_to_child . 'header.tpl.php' ) ? $path_to_child : $path;

  return array(
    'cogito_header' => array(
     'template' => 'header',
     'render element' => 'cogito_header',
     'path' => $path,
    ),
  );
}