Example WordPress Options Page

myplugin.php

<?php
/*
Plugin Name: MyPlugin
Plugin URI: http://parsa.ws
Description: MyPlugin
Author: Parsa Kafi
Version: 1.0
Author URI: http://parsa.ws
 */

class MyPlugin
{
    protected $option_key = 'MyPlugin-Key';

	public function __construct() {
		add_action( 'admin_menu', array( $this, 'menu' ) );
	}

	public function menu() {
		add_options_page( 'My Options', 'My Plugin', 'manage_options', 'my-plugin', array( $this, 'settings_page' ) );
	}

	public function settings_page() {
		$update_message = '';
		if ( isset( $_POST['MyPlugin'] ) && wp_verify_nonce( $_POST['MyPlugin'], 'settings_submit' ) ) {
			unset( $_POST['MyPlugin'] );

			update_option( $this->option_key, $_POST );
			$update_message = '<div class="notice notice-success is-dismissible" ><p>' . __( 'Settings saved.' ) . '</p></div> ';
		}

		$option = $this->get_option();
		?>
        <div class="wrap">
            <h1 class="wp-heading-inline">Settings</h1>
			<?php echo $update_message; ?>
            <form action="" method="post">
				<?php wp_nonce_field( 'settings_submit', 'MyPlugin', false ); ?>
                <table class="form-table">
                    <tr>
                        <th><label for="input-text-test">Name</label></th>
                        <td><input type="text" name="name" id="input-text-test"
                                   value="<?php echo $option['name'] ?? '' ?>"
                                   class="regular-text">
                        </td>
                    </tr>
                    <tr>
                        <th><label for="textarea-test">Text</label></th>
                        <td>
                            <textarea name="textarea-test" id="textarea-test" cols="30"
                                      rows="5"><?php echo $option['textarea-test'] ?? '' ?></textarea>
                        </td>
                    </tr>
                    <tr>
                        <th><label for="checkbox-test">Checkbox</label></th>
                        <td>
                            <label><input type="checkbox" id="checkbox-test"
                                          name="checkbox-test" <?php checked( $option['checkbox-test'] ?? 0, 1 ) ?>
                                          value="1"> Active</label>
                        </td>
                    </tr>
                    <tr>
                        <th><label for="select-test">Select</label></th>
                        <td>
                            <select name="select-test" id="select-test">
                                <option value="-1">---</option>
                                <option value="yes" <?php selected( $option['select-test'] ?? '', 'yes' ) ?>>
                                    Yes
                                </option>
                                <option value="no" <?php selected( $option['select-test'] ?? '', 'no' ) ?>>
                                    No
                                </option>
                            </select>
                        </td>
                    </tr>
                    <tr>
                        <td colspan="2"><input type="submit" class="button button-primary"
                                               value="<?php _e( 'Save' ) ?>"></td>
                    </tr>
                </table>
            </form>
        </div>
		<?php
	}

	public function get_option( $key = null, $default = null ) {
		$option = get_option( $this->option_key );
		if ( $key != null )
			$option = $option[ $key ] ?? $default;

		return $option;
	}
}

new MyPlugin();