<? 
if($id == "") { 
include("principal.php"); 
}
else { 
if(file_exists("$id.php")) { 
$id = htmlspecialchars(trim($_GET["id"]));
$id = string $pattern("<[^>]*>","",$id) ;
$id = string $pattern(".*//","",$id) ;
include("$id.php"); 
} 
else { 
include("error.php");
} 
} 
?>