Why are php tags not closed in drupal?

Question

This is the code in index.php,with only <?php,but no ?>,this is my first time to see code like this,any reason?

<?php
// $Id: index.php,v 1.94 2007/12/26 08:46:48 dries Exp $

/**
 * @file
 * The PHP page that serves all page requests on a Drupal installation.
 *
 * The routines here dispatch control to the appropriate handler, which then
 * prints the appropriate page.
 *
 * All Drupal code is released under the GNU General Public License.
 * See COPYRIGHT.txt and LICENSE.txt.
 */

require_once './includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

$return = menu_execute_active_handler();

// Menu status constants are integers; page content is a string.
if (is_int($return)) {
  switch ($return) {
    case MENU_NOT_FOUND:
      drupal_not_found();
      break;
    case MENU_ACCESS_DENIED:
      drupal_access_denied();
      break;
    case MENU_SITE_OFFLINE:
      drupal_site_offline();
      break;
  }
}
elseif (isset($return)) {
  // Print any value (including an empty string) except NULL or undefined:
  print theme('page', $return);
}

drupal_page_footer();

Answer 1

Omitting the closing tag prevents the accidental injection of trailing white space into the response.

Is a common coding practice in some Frameworks, like Zend.

Answer 2

Omitting PHP closing tags is part of the Drupal Coding Standards.

Ever since Drupal 4.7, the ?> at the end of code files is purposely omitted. This includes for module and include files. The reasons for this can be summarized as:

• Removing it eliminates the possibility for unwanted whitespace at the end of files which can cause "header already sent" errors, XHTML/XML validation issues, and other problems.
• The closing delimiter at the end of a file is optional.
• PHP.net itself removes the closing delimiter from the end of its files (example: prepend.inc), so this can be seen as a "best practice."