如何仿写thinkphp的C方法？

<?php
return array(
             ‘db_user‘=>‘root‘,
             ‘db_pass‘=>‘root‘,
             ‘db_name‘=>‘test‘,
             ‘db_charset‘=>‘utf8‘,
             );
?>

<?php
$username=C("db_user");
var_dump($username);
function C($key)
{
   $list=include ‘config.php‘;
   return empty($list[$key])?null:$list[$key];
}
?>