Paginated output in PHP

<?php                    
if (!empty($_GET['do']) && file_exists( "action/" . $_GET['do'] . '.php' ) )
{
  include "action/" . $_GET['do'] . '.php';
}
else  
{
include "action/checking.php"; 
}
?>

<a href="?do=main" class="button">CLICK</a><br>