PHP MVC from the scratch - How to connect these classes?
<?php
// filename: "config.php"

// another includes or requires here

class Config
{
   // begin data section
   public static $UserName;
   public static $UserPassword;
   // end data section

   // begin singleton section
   private static $_instance;

   public static function getInstance()
   {
      if (!self::$_instance instanceof self)
      {
         self::$_instance = new self;
      }
      return self::$_instance;
   }
   // end singleton section

   // start session section
   public static function SaveSession()
   {
     $_SESSION['config_username'] =  Config::$UserName;
     $_SESSION['config_password'] =  Config::$UserPassword;
   }

   public static function RestoreSession()
   {
     Config::$UserName = $_SESSION['config_username'];
     Config::$UserPassword = $_SESSION['config_password'];
   }
   // end session section

} // class Config

?>

<?php
// filename: "index.php"

include ("config.php");
// another includes or requires here

class Application
{
   public static function main()
   {
      // prepare session variables
      session_start();

      // prepare singletons here
      $instance = Config::getInstance();

      // this code its an example
      $instance->UserName = "johndoe";
      $instance->UserPassword = "123";
      $instance->SaveSession();

      // all the page construction goes here
      // ...
   }
} //

// program starts here:
Application::main();

?>

<?php
// filename: "customers.php"

include ("config.php");
// another includes or requires here

class Application
{
   public static function main()
   {
      // copy data from session to singletons
      $instance->RestoreSession();

      // all the page construction goes here
      // ...
   }
} //

// program starts here:
Application::main();

?>