Sample WP Plugin that supports admin options

test-plugin.php

<?php
/**
 * Plugin Name: AAA Test Plugin
 * Description: A sample plugin that supports admin options.
 * Author: Matt Thomas
 * Version: 0.0.1
 *
 * @package WordPress
 */

/**
 * Add link to this plugin's settings page
 *
 * @param [type] $links
 * @return void
 */
$plugin = plugin_basename( __FILE__ );
add_filter(
	"plugin_action_links_$plugin", function( $links ) {
		$settings_link = '<a href="options-general.php?page=aaa-test-plugin">' . __( 'Settings' ) . '</a>';
		array_push( $links, $settings_link );
		return $links;
	}
);

/**
 * Callback that adds options to the settings page.
 *
 * @return void
 */
function plugin_options_page() {
	?>
	<div>
	<h2>My custom plugin</h2>
	Options relating to the Custom Plugin.
	<form action="options.php" method="post">
	<?php settings_fields( 'plugin_options' ); ?>
	<?php do_settings_sections( 'plugin' ); ?>
	 
	<input name="Submit" type="submit" value="<?php esc_attr_e( 'Save Changes' ); ?>" />
	</form></div>
	 
	<?php
}

/**
 * Adds sub-menu to settings admin menu item
 *
 * @return void
 */
add_action(
	'admin_menu', function () {
		add_options_page(
			$menu_title = 'Custom Plugin Menu', // Link text under settings.
			$page_title = 'Custom Plugin Page', // Plugin page title.
			$capability = 'manage_options', // User must have this capability.
			$menu_slug  = 'aaa-test-plugin', // Plugin slug (unique alias).
			$function   = 'plugin_options_page' // Function that generates plugin options.
		);
	}
);

/**
 * Outputs text for the main settings
 *
 * @return void
 */
function plugin_section_text() {
	echo 'Foo bar baz qux!';
}

/**
 * Outputs the setting field.
 *
 * @return void
 */
function plugin_setting_string() {
	$options = get_option( 'plugin_options' );
	echo "<input id='plugin_text_string' name='plugin_options[text_string]' size='40' type='text' value='{$options['text_string']}' />";
}

add_action(
	'admin_init', function () {
		register_setting(
			'plugin_options', // Group ID, same as settings_fields() value.
			'plugin_options', // Options name.
			'plugin_options_validate' // Function callnack to validate options.
		);

		add_settings_section(
			'plugin_main', // Section unique ID.
			'Main Settings', // Rendered section title.
			'plugin_section_text', // Function callback to output section text.
			'plugin' // Page name, must match do_settings_sections() value.
		);

		add_settings_field(
			'plugin_text_string', // Field unique ID.
			'Plugin Text Input', // Field title.
			'plugin_setting_string', // Function callback to output the field.
			'plugin', // Page name this is attached to (same as do_settings()).
			'plugin_main' // ID of the settings section this is part of (same as add_settings_section() first argument).
		);
	}
);