you can use javascript, why not
if( $myrow )
{
$this->_setSession( $username, $myrow[2], $myrow[3] );
echo "<body><script language='javascript'>
wnd = window.open('popup.php?foo=bar&action=act1',
'tpopupwindow',
'toolbar=no,location=no,directoties=no,'+
'status=no,menubar=no,scrollbars=yes,resizable=yes,'+
'width=300,height=200,left=150,top=100');
</script></body>";
exit;
}
else
{
header("location: ../error/failedLogin.php");
}