<?php add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position ); ?>

function create_post_type() {
  register_post_type( 'client_posttype',
    array(
      'labels' => array(
        'name' => __( 'Clients' ),
        'singular_name' => __( 'Client' )
      ),
      'public' => true,
      'has_archive' => true,
    )
  );
}
add_action( 'init', 'create_post_type' );

class options_page {

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

    function admin_menu() {
        add_menu_page( 'Clients', 'Clients Page', 'edit_posts', 'clients_page', 'my_clients', '', 24);
    }

    function  settings_page() {
        echo 'This is the page content';
    }
}

new options_page;