PHP.EXE was used as standalone, with php_interbase.dll configured as module.

Trying to connect/query to InterBase database with:
   $dbh = ibase_connect(xxx, xxx, xxx);
   $stmt = 'SELECT * FROM customer';
   $sth = ibase_query ($dbh, $stmt);
   while ($row = ibase_fetch_object ($sth)) 
   {
     print $row->co_name;
   }
  ibase_close ($dbh);

If ibase_connect fails, and ibase_query gets executed, PHP.EXE will crash. Following was captured as output from PHP.EXE before crash:
Warning:  0 is not a valid InterBase link resource in xxx.php on line 17

Where line 17 is ibase_query.