SelectForm.php

<?php

namespace Drupal\theming_example\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * A simple form that displays a select box and submit button.
 *
 * This form will be be themed by the 'theming_example_select_form' theme
 * handler.
 */
class SelectForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'theming_example_form_select';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $options = [
            'newest_first' => $this->t('Newest first'),
            'newest_last' => $this->t('Newest last'),
            'edited_first' => $this->t('Edited first'),
            'edited_last' => $this->t('Edited last'),
            'by_name' => $this->t('By name'),
        ];
        $form['choice'] = [
            '#type' => 'select',
            '#options' => $options,
            '#title' => $this->t('Choose which ordering you want'),
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Go'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addMessage($this->t('You chose %input', [
            '%input' => $form_state->getValue('choice'),
        ]));
    }

}