settings.php

<?php

class Runscope_Settings {

	public function __construct() {
		if ( is_admin() ) {
			add_action( 'admin_menu', array( $this, 'add_plugin_page' ) );
			add_action( 'admin_init', array( $this, 'page_init' ) );
		}
	}

	public function add_plugin_page(){
		// This page will be under "Settings"
		add_options_page( 'Runscope Settings', 'Runscope Settings', 'manage_options', 'runscope-settings-admin', array( $this, 'create_admin_page' ) );
	}

	public function create_admin_page() {
		?>
		<div class="wrap">
			<?php screen_icon(); ?>
			<h2>Settings</h2>
			<form method="post" action="options.php">
				<?php
				settings_fields( 'runscope_option_group' );
				do_settings_sections( 'runscope-settings-admin' );
				submit_button();
				?>
			</form>
		</div>
		<?php
	}

	public function page_init() {
		register_setting( 'runscope_option_group', 'array_key', array( $this, 'check_ID' ) );

		add_settings_section(
			'setting_section_id',
			'Bucket Settings',
			array( $this, 'print_section_info' ),
			'runscope-settings-admin'
		);

		add_settings_field(
			'runscope_bucket_id',
			'Bucket ID',
			array( $this, 'create_an_id_field' ),
			'runscope-settings-admin',
			'setting_section_id'
		);
	}

	public function check_ID( $input ) {
		$mid = $input['runscope_bucket_id'];

		if ( get_option( 'runscope_bucket_id' ) === FALSE ) {
			add_option( 'runscope_bucket_id', $mid );
		} else {
			update_option( 'runscope_bucket_id', $mid );
		}

		return $mid;
	}

	public function print_section_info(){
		print 'Enter your bucket ID below:';
	}

	public function create_an_id_field(){
		?>
		<input type="text" id="bucket" name="array_key[runscope_bucket_id]" value="<?php echo get_option( 'runscope_bucket_id' ); ?>" />
		<?php
	}
}

$runscope_settings = new Runscope_Settings();