try this code with a valid default.xml with some xml code.
it works fine with php3 and not with php4....
i get it to work with php4 too by adding " to $tgname as in the commented line.
i think it's a bug.... or it should be documented in the incompatibility list...

<?
 $level = 0;
 $tagname = array();

 function XMLFstart($parser, $tgname, $attrs) {
  $GLOBALS['level']++;
  $GLOBALS['tagname'][$GLOBALS['level']] = $tgname;
#  $GLOBALS['tagname'][$GLOBALS['level']] = "$tgname";
  print('&lt;'.$GLOBALS['tagname'][$GLOBALS['level']].'&gt;');
 }

 function XMLFend($parser, $tgname) {
  $GLOBALS['level']--;
  print('&lt;/'.$GLOBALS['tagname'][$GLOBALS['level']].'&gt;');
 }
 
 function XMLFparse($file) {
  $xml_parser = xml_parser_create();
  xml_set_element_handler($xml_parser, "XMLFstart", "XMLFend");
  xml_parser_set_option($xml_parser, XML_OPTION_CASE_FOLDING, true);

  $fp = fopen($file, "r");
  while ($data = fread($fp, filesize($file))) {
   if (!xml_parse($xml_parser, $data, feof($fp))) {
    die(sprintf("<BR><BR>XML error: %s at line %d",xml_error_string(xml_get_error_code($xml_parser)),xml_get_current_line_number($xml_parser)));
   }
  }
  xml_parser_free($xml_parser);
  fclose($fp);
 }

 XMLFparse('./default.xml');
?>