I installed Debian Woody/PHP 4 on a clean machine, and apt-get dist-upgrade'd to the latest files.  Ereg and ereg_replace will cause the page to fail.  Here's an example program:

<?
   $a = "Hello Mars!";
   $a = ereg_replace("Mars", "World", $a);
   echo $a;
?>

When I load this page in IE 5.0, I get a "The Page Cannot Be Displayed" error.  If I comment out the ereg line it works fine.