plugin-menu-wp.php

<?php
/*
    Plugin Name: Our Plugin Menu
	Description: Adding Sub menu in admin
	Version: 1.0
	Author: Bam
	Author URI: https://wpenginer.com
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

class OurPluginMenu {
	function __construct() {
		add_action( 'admin_menu', array( $this, 'ourMenu' ) );
	}

	function ourMenu() {
		add_menu_page( 'Our Plugin Menu', 'Our Plugin Menu', 'manage_options', 'our-plugin-menu', array(
			$this,
			'optionsMainPage'
		), 'dashicons-smiley', 2 );
		add_submenu_page( 'our-plugin-menu', 'Sub Menu Default Page', 'Sub Menu Default', 'manage_options', 'our-plugin-menu', array(
			$this,
			'optionsMainPage'
		), 1 );
		add_submenu_page( 'our-plugin-menu', 'Sub Menu Options', 'Sub Menu', 'manage_options', 'sub-menu-options', array(
			$this,
			'optionsSubPage'
		), 2 );
	}

	function handleForm() {
		if ( wp_verify_nonce( $_POST['ourNonce'], 'saveInput1' ) and current_user_can( 'manage_options' ) ) {
			update_option( 'input1', $_POST['input1'] ); ?>
            <span>You have updated the input</span>
		<?php } else { ?>
            <span>You don't have permission</span>
		<?php }
	}

	function optionsMainPage() { ?>
        <div class="wrap">
            <h1><?php esc_html_e( get_admin_page_title() ); ?></h1>
			<?php if ( $_POST['justsubmitted'] ) {
				$this->handleForm();
			} ?>
            <form method="post">
                <input type="hidden" name="justsubmitted" value="true">
				<?php wp_nonce_field( 'saveInput1', 'ourNonce' ) ?>
                <table class="form-table">
                    <tbody>
                    <tr>
                        <th scope="row">
                            <label for="input1">Input1</label>
                        </th>
                        <td><input id="input1" name="input1" type="text"
                                   placeholder="<?php esc_attr_e( get_option( 'input1' ) ); ?>"/></td>
                    </tr>
                    </tbody>
                </table>
                <p class="submit"><input type="submit" name="submit" id="submit" class="button button-primary"
                                         value="Save Changes"></p>
            </form>
        </div>
	<?php }

	function optionsSubPage() { ?>
        Hello World form Option Sub Page
	<?php }
}

$ourPluginMenu = new OurPluginMenu();