IvanDanilov

Lietuva:

<? include_once('core/' . $_GET['module'] . '.php' ); ?>

Rusija:

<? include_once('core/' . str_replace("/", "", $_GET['module']) . '.php' ); ?>

USA:

<? 
  $modules = [];
  if ( $handle = opendir("core") ) {
     while ( $entry = readdir($handle) )
        if ( is_file($entry) )
            $modules[] = str_replace( ".php", "", $entry) );
     closedir($handle);
  }
  if ( isset($_GET['module']) && in_array($_GET['module'], modules) ) 
     include_once('core/' . $_GET['module'] . '.php');
  else
     exit();
?>

All the above written is a bad code