Class not found, composer and Zend Framework 1 autoloader issue

I have the following class autoload definition in a [root]/composer.json file:

{
  ...
  "autoload": {
    "psr-0": {
      "": [
        "application/models",
        "application/controllers",
        "application/forms",
        "library/"
      ]
    },
    "psr-4": {
      "": ["src/"]
    },
    "classmap": [
      "app/AppKernel.php",
      "app/AppCache.php"
    ]
  },
  ...
}

When I call the [root]/public_html/index.php page I got the following error:

PHP Fatal error: Uncaught Error: Class 'classes\DependencyInjection' not found in /var/www/html/application/bootstrap.php:29

What's in [root]/public_html/index.php is the following code:

$bootstrap = true;
require_once '../application/bootstrap.php';

And what's in [root]/application/bootstrap.php file is:

// turn on autoloading for classes
// composer autoloader
include(MMIPATH.'/vendor/autoload.php');

// zend autoload
require_once 'Zend/Loader/Autoloader.php';
Zend_Loader_Autoloader::getInstance();

$diContainer = new classes\DependencyInjection(services.yaml');
$proxy       = $diContainer->get('containerProxy');

This is the definition of [root]/library/classes/DependencyInjection.php:

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\Config\FileLocator;
use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
USE Symfony\Component\DependencyInjection\Container;

class DependencyInjection extends ContainerBuilder
{
    ....
}

What's wrong here? Why autoloader can't find that class?

Solution

You're attempting to load a "classes" namespace, however your class is not defined as being in the "classes" namespace.

new classes\DependencyInjection(...) in PSR-0 loads {paths}\classes\DependencyInjection.php and attempts to instantiate the class DependencyInjection from the namespace classes, but DependencyInjection is not in the classes namespace. The file will load but the class does not exist.

You could add namespace classes; to each of these classes, but that's not exactly a good solution. Better solution is to use proper namespacing or change your PSR-0 list to include library/classes and use new DependencyInjection(...). (My vote is for the first one -- use proper namespaces.)

As requested. example:

File Location
{app}\library\UsefullNamespace\DependencyInjection.php

Call it using new UsefullNamespace\DependencyInjection.php

DependencyInjection.php:

namespace UsefullNamespace;  

use [...];  

class DependencyInjection extends ContainerBuilder
{