<?
if(isSet($content) and isSet($id))
{
?>
<frameset cols="150,*">
 <frame name="left" src="index.php?id=<?=$id;?>"></frame>
 <frame name="content" src="<?=$content;?>"></frame>
</frameset>
<?
}
if(isSet($id))
{
 echo "<a href=\"index.php?id=1&content=main.php\" target=\"main\">Main</a>
";
 if($id == 1)
  {
   echo "<a href=\"#\" target=\"content\">Main 2</a>
";
   echo "<a href=\"#\" target=\"content\">Main 3</a>
";
  } 
 echo "<a href=\"index.php?id=2&content=links.php\" target=\"main\">Links</a>
";
 if($id == 2)
  {
   echo "<a href=\"#\" target=\"content\">Links 2</a>
";
   echo "<a href=\"#\" target=\"content\">Links 3</a>
";
  } 
}
if(empty($content) and empty($id))
{
?>
<frameset>
 <frame name="main" src="index.php?content=start.php&id=0"></frame>
</frameset>
<?
}

?>