Description:
------------
When using require_once("file.php") to include "file.php" in multiple files, files which are included in each another on several levels, require_once("file.php") randomly fails to include the file at all.
Most of the time it includes it, but sometimes it will just fail and the system will come to a halt since that file is vital.

Changing to require("file.php") works, but obviously this would include the file several times.

Reproduce code:
---------------
inc.php

$dsn = 'mssql://user:pass@localhost/database';
$db = DB::connect($dsn);
if( PEAR::isError( $db ) )
{
   die "Database connect failed";
}

file1.php:

require_once("inc.php");

file2.php
require_once("file1.php");
require_once("inc.php");

Expected result:
----------------
inc.php included once every timed

Actual result:
--------------
inc.php sometimes is not included at all, since the database connection in inc.php is not initialized and $db is NULL