Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try avoid embedding huge scripts into the report.

More for code :

// file base.inc
<?php
class Base
{ var $test;
  var $info;
 function test()
  { //...anything..
  }
}
?>
//this is other file
<?php
require_once("base.inc");
class system_login extends Base
{
   private $db_conn; 
   private $user_table = 'user';
  function system_login( $DB = "", $mode = "http" )
  {
  global $Global_DB_Connection;
   if (empty($DB)) 
    $this->db_conn = $Global_DB_Connection;
   else
    $this->db_conn = $DB;
   if ($mode == 'http')
    {
    if (!isset($_SERVER['PHP_AUTH_USER'])) {
   header('WWW-Authenticate: Basic realm="My Realm"');
   header('HTTP/1.0 401 Unauthorized');
   echo 'please login at first !';
   exit;
  } else {
   Base::sys_login($_SERVER['PHP_AUTH_USER'],$_SERVER['PHP_AUTH_PW']);
   // ...anything..
  }
  }
 else
{
//... something for Form submit to login ...
}  
}
}

$a = new system_login ("","Form");
?>

if changed the :
function system_login( $DB = "", $mode = "http" )
to
function system_login( $DB, $mode )
used the
$a = new system_login ("","Form");
or
$a = new system_login ("","");

The PHP5 told me all the parameters missing !!

if I used an object to $DB , like :
$a = new system_login ($Global_DB_Connection, "Form");
I found the "Form" can't transfer to $mode also !

Can't reproduce it with your code. Works fine.

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".