Drupal: Set header data (title, metatags).

foo.php

<?php
// Insert into page callback.
drupal_add_html_head(array(
  '#tag' => 'title',
  '#value' => 'new header title',
), 'foo_title');

drupal_add_html_head(array(
  '#tag' => 'meta',
  '#attributes' => array(
    'name' => 'description',
    'content' => 'hello world',
  ),
), 'foo_meta');