// The allowed extensions
$ext = array(".php",".jpg");

// Try every extension
foreach($ext as $extension)
{
  // Build the id
  $id = $id.$extension;
  // Check if the file actually exists
  if (file_exists($id))
  {
    // Include the file and make sure we know that it is included
    include($id);
    $included=true;
  }
}

// If we have included something, 
// don't include the main.php any more
if (!$included)
{
  // Include the default main.php
  include("main.php");
}