Type#1:
<?php
require_once('Second.php');
require_once('First.php');
$Second = new Second;
$Second->run_second();
$First = new First;
$First->run_first();
?>
---------------------------------------------------------
Type#2:
<?php
spl_autoload_register(function($class){
//
require_once($class.'.php');
});
$Second = new Second;
$Second->run_second();
$First = new First;
$First->run_first();
?>