
<?php

class Foo {

	[...]

    /**   
     * Whitelist the Foo options
     *
     * @return none
     */
    function register_settings () {
        register_setting( 'Foo_options' , 'Foo', array( &$this, 'merge_options' ) );
    }

	/**
	 * Preserve options when using multiple pages
	 *
	 * @return merged data
	 */
    function merge_options( $data ) {
        $existing = $this->options; // this contains get_option( 'Foo' );
        if ( !is_array( $existing ) || !is_array( $data ) ) // something went wrong
            return $data;
        return array_merge( $existing, $data );
    }
}
?>