First of all, we need to require Doctrine through Composer. At the root folder of the project, the command below has to be executed:

 > php composer.phar require "doctrine/orm=^2.5"

Then, these lines will allow you to set up Doctrine:

require_once '/path/to/vendor/autoload.php';

use DoctrineORMToolsSetup;
use DoctrineORMEntityManager;

$paths = ['/path/to/entity-files'];
$isDevMode = false;

// the connection configuration
$dbParams = [
    'driver'   => 'pdo_mysql',
    'user'     => 'the_database_username',
    'password' => 'the_database_password',
    'dbname'   => 'the_database_name',
];

$config = Setup::createAnnotationMetadataConfiguration($paths, $isDevMode);
$entityManager = EntityManager::create($dbParams, $config);