Drupal - How to get $_POST and $_GET parameters

From the change record:

$name = $_POST['name']; // form param

becomes

$name = \Drupal::request()->request->get('name'); // form param

Incidentally, for GET vars, you would use:

$query = \Drupal::request()->query->get('name');

In a controller get the request with a type-hinted argument Request $request:

<?php

namespace Drupal\mymodule\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\Request;

class ExampleController extends ControllerBase {

  /**
   * Controller to return a POST or a GET parameter.
   */
  public function action(Request $request) {

    // get your POST parameter
    $foo = $request->request->get('foo');

    // or get your GET parameter
    $foo = $request->query->get('foo');

    // POST requests are not cached, but for GET you need a cache context
    return [
      '#markup' => $foo,
      '#cache' => ['contexts' => ['url.query_args:foo']],
    ];
  }

}

More info https://www.drupal.org/docs/8/api/routing-system/using-parameters-in-routes

Inject the RequestStack into your controller.

The current request contains the query attribute that in turn contains the GET parameters. request contains the POST parameters.

<?php

namespace Drupal\example_module\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\RequestStack;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * An example controller.
 */
class ExampleController extends ControllerBase {

  /**
   * @var Symfony\Component\HttpFoundation\RequestStack
   */
  private $requestStack;

  /**
   * Constructor.
   *
   * @param Symfony\Component\HttpFoundation\RequestStack $request_stack
   */
  public function __construct(RequestStack $request_stack) {
    $this->requestStack = $request_stack;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('request_stack')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function action() {
    // Get your GET parameter here.
    $this->requestStack->getCurrentRequest()->query->get('foo');
  }

}