I have tried installing OMNIHTTPd 2.04 PRO on two computers with Win98SE and PHP3.0.11
I always get:

Warning: not a valid database identifier 0 in F:\HTTPD\HTDOCS\main.php3 on line 122

Here is a short script that would be enough to cause this error with the php3_dbm.dll extension loaded:

<script language=php>
$d = "D:\httpd\HtDocs\database\newsdb";
$db = dbmopen($d, "r");

$totalarts = dbmFetch($db,  "highstory");
</script>