Не подключается к MySQL?

<?php 
index.php
 if($_SERVER['REQUEST_URI'] == '/')
     $page = 'home';
  else
      $page = substr($_SERVER['REQUEST_URI'], 1); 

session_start();

include 'config.php';

 if(file_exists("all/$page.php"))
     include "all/$page.php";

  else if ($_SESSION['id'] == 1 and file_exists("auth/$page.php"))
      include "auth/$page.php";

  else if ($_SESSION['id'] == 1 and file_exists("guest/$page.php"))
      include "guest/$page.php";

  else
      exit('404');
?>
config.php
<code lang="php">
<?php

define('DB_HOST', 'localhost');
define('DB_USER', 'mysql');
define('DB_PASSWORD', 'mysql');
define('DB_NAME', 'logi1k');

function db() {
	global $db;
	$db = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
	if (!$db)
		exit('Ошибка подключеня к БД');
}

?>
</code>