The option XML_OPTION_SKIP_WHITE has no effect in my PHP 5.2.6 (with expat-1.95.8-5). To skip cdata composed of white space only, simply check for that at the beginning of your cdata callback function:

<?php
function callback_cdata($parser, $cdata)
{
 if(!trim($cdata))
  return;

 // ... continue processing ...
}
?>