How to Setup Your First Wordpress Plugin?

admin-settings.php

<h1>Very Simple Plugin Settings</h1>

<form id="vsp_settings_form" method="POST" action="<?= esc_url(admin_url('admin-post.php')); ?>">
  <input type="hidden" name="action" value="vsp_update_option">
  <label for="vsp_text_value">Highly Complicated Text</label>
  <input type="text" name="vsp_text_value" id="vsp_text_value">
  <?= submit_button("Submit") ?>
</form>

very-simple-plugin.php

<?php
/**
 * Plugin Name: My Very Simple Plugin
 * Description: It does very simple things
 * Author: Simple Man
 */

if (!defined('WPINC')) {
	die;
}

define('PLUGIN_TITLE', 'Very Simple Plugin'); 
define('PLUGIN_TEXT_OPTION', 'vsp_plugin_text_option');

function vsp_setup_admin_page() {
  require_once plugin_dir_path(__FILE__) . 'views/admin-settings.php';
}

function vsp_setup_admin_menu() {
  add_menu_page(PLUGIN_TITLE, PLUGIN_TITLE, 'manage_options', 'vsp_admin_page', 'vsp_setup_admin_page');
}

function vsp_handle_update_option() {
  try {
    $value = sanitize_text_field($_POST['vsp_text_value']);
    update_option(PLUGIN_TEXT_OPTION, $value);
    wp_safe_redirect(
      add_query_arg(array(
        'page' => 'vsp_admin_page',
        'success' => true,
      ), admin_url('admin.php'))
    );
  } catch (Exception $e) {
    wp_safe_redirect(
      add_query_arg(array(
        'page' => 'vsp_admin_page',
        'success' => false
      ), admin_url('admin.php'))
    );
  }
}

function vsp_admin_notice() {
  if (!isset($_GET['success'])) {
    return;
  }

  if ($_GET['success'] === 'false') {
    echo '<div class="notice notice-error"><p>Something went wrong</p></div>';
    return;
  }
  echo '<div class="notice notice-success"><p>Successfully updated settings!</p></div>';
  return;
}

function vsp_display_option_shortcode() {
  return get_option(PLUGIN_TEXT_OPTION);
}

add_action('admin_menu', 'vsp_setup_admin_menu');
add_action('admin_post_vsp_update_option', 'vsp_handle_update_option');
add_action('admin_notices', 'vsp_admin_notice');
add_shortcode('vsp_text', 'vsp_display_option_shortcode');