Example : (Insert this line in to your form submitForm() methode)

Redirect to the user page ( Redirection on form submit)

$form_state->setRedirect('user.page');

Example :  Display user login form on a page

Step 1:  Create a module. (Example)

Example:



  $con = Drupal\Core\Database\Database::getConnection();

  $start_value = 100000;

  $query = "ALTER TABLE {the_table} AUTO_INCREMENT = $start_value";

  $con->query($query);

Render arrays