PSR-0vsPSR-4

##PSR-0
  $ composer init
  $ mkdir -p vendor/rar/form/src/Form/Controller
  $ nano vendor/rar/form/src/Form/Controller/Fields.php
    <?php

  $ namespace Form\Controller;


    class Fields
    {
            public function __construct()
            {
                    die ("I'm Fields");
            }
    }

  $ nano composer.json
      {
          "name": "root/psr",
          "require": {},
          "autoload":{
              "psr-0":{
              "Form\\Controller":"vendor/rar/form/src"

              }
          }
      }

  $ composer --list | grep  autoload
  $ composer dump-autoload -o

  $ nano index.php
        <?php

        require __DIR__."/vendor/autoload.php";

        $form = new Form\Controller\Fields();

##PSR-4
  $ mv vendor/rar/form/src/Form/Controller/Fields.php vendor/rar/form/src/Fields.php

  "psr-4":{
        "Form\\Controller\\":"vendor/rar/form/src"