some-plugin-admin.php

<?php

class some_plugin_admin {
	public static function plugins_loaded() {
		add_action( 'admin_init', 'some_plugin_admin::admin_init' );
		add_action( 'admin_menu', 'some_plugin_admin::admin_menu' );
	}

	public static function admin_init() {
		register_setting( 'some_plugin', 'some_plugin_options' );

		add_settings_section( 'some_plugin_general', __( 'General', 'some_plugin' ), 'some_plugin_admin::render_general', 'some_plugin' );
		add_settings_field( 'some_plugin_number', __( 'Footer Number', 'some_plugin' ), 'some_plugin_admin::render_number', 'some_plugin', 'some_plugin_general' );
	}

	public static function admin_menu() {
		add_options_page( __( 'Some Plugin', 'some_plugin' ), __( 'Some Plugin!', 'some_plugin' ), 'publish_posts', 'some_plugin', 'some_plugin_admin::render' );
	}

	public static function render() {
		?>
		<div class="wrap" id="some-plugin-admin">
			<h2><?php _e('Some Plugin', 'some_plugin'); ?></h2>
			<form action="options.php" method="POST">
				<?php settings_fields('some_plugin'); ?>
				<?php do_settings_sections('some_plugin'); ?>
				<?php submit_button(); ?>
			</form>
		</div>
		<?php
	}

	public static function render_general() {
		?>
		<p><?php _e( 'Configure Some Plugin.', 'some_plugin' ); ?></p>
		<?php
	}

	public static function render_number() {
		?>
		<input id="some_plugin_number" name="some_plugin_options[number]" type="number" value="<?php echo esc_attr( some_plugin_options::get( 'number', 1 ) ); ?>" />
		<?php
	}
}

some-plugin-footer.php

<?php

class some_plugin_footer {
	public static function plugins_loaded() {
		add_action( 'wp_footer', 'some_plugin_footer::wp_footer' );
	}

	public static function wp_footer() {
		$number = some_plugin_options::get( 'number', 0 );
		printf( "The number is %d.", $number );
	}
}

some-plugin-options.php

<?php

class some_plugin_options {
	private static $options;

	private static function load() {
		if ( ! isset( self::$options ) )
			self::$options = get_option( 'some_plugin_options' );

		if ( ! is_array( self::$options ) )
			self::$options = array();
	}

	public static function save() {
		update_option( 'some_plugin_options', self::$options );
	}

	public static function get( $name, $default ) {
		self::load();

		if ( self::has( $name ) )
			return self::$options[$name];

		return $default;
	}

	public static function has( $name ) {
		self::load();

		return isset( self::$options[$name] );
	}

	public static function set( $name, $value ) {
		self::load();

		self::$options[$name] = $value;

		self::save();
	}

	public static function delete( $name ) {
		self::load();

		unset( self::$options[$name] );

		self::save();
	}
}

some-plugin.php

<?php
/*
Plugin name: Some Plugin
*/

if ( function_exists( 'add_action' ) )
	add_action( 'plugins_loaded', 'some_plugin::plugins_loaded' );

class some_plugin {
	public static function load_classes() {
		$path = dirname( __FILE__ ) . '/';
		require( $path . 'some-plugin-admin.php' );
		require( $path . 'some-plugin-footer.php' );
		require( $path . 'some-plugin-options.php' );
	}

	public static function plugins_loaded() {
		some_plugin_admin::plugins_loaded();
		some_plugin_footer::plugins_loaded();
	}
}

some_plugin::load_classes();