public static function register_pages() {

		$admin_pages = apply_filters( 'pum_admin_pages', array(
			'subscribers' => array(
				'page_title'  => __( 'Subscribers', 'popup-maker' ),
				'capability'  => 'manage_options',
				'callback'    => array( 'PUM_Admin_Subscribers', 'page' ),
			),
			'settings'   => array(
				'page_title'  => __( 'Settings', 'popup-maker' ),
				'capability'  => 'manage_options',
				'callback'    => array( 'PUM_Admin_Settings', 'page' ),
			),
			'extensions' => array(
				'page_title'  => __( 'Upgrade', 'popup-maker' ),
				'capability'  => 'edit_posts',
				'callback'    => array( 'PUM_Admin_Extend', 'page' ),
			),
			'support'    => array(
				'page_title'  => __( 'Help & Support', 'popup-maker' ),
				'capability'  => 'edit_posts',
				'callback'    => array( 'PUM_Admin_Support', 'page' ),
			),
			'tools'      => array(
				'page_title'  => __( 'Tools', 'popup-maker' ),
				'capability'  => 'manage_options',
				'callback'    => array( 'PUM_Admin_Tools', 'page' ),
			),
		) );

		foreach ( $admin_pages as $key => $page ) {
			$page = wp_parse_args( $page, array(
				'parent_slug' => 'edit.php?post_type=popup',
				'page_title'  => '',
				'menu_title'  => '',
				'capability'  => 'manage_options',
				'menu_slug'   => '',
				'callback'    => '',
			) );

			// Backward compatibility.
			$page['capability'] = apply_filters( 'popmake_admin_submenu_' . $key . '_capability', $page['capability'] );

			if ( empty( $page['menu_slug'] ) ) {
				$page['menu_slug'] = 'pum-' . $key;
			}

			if ( ! empty( $page['page_title'] ) && empty( $page['menu_title'] ) ) {
				$page['menu_title'] = $page['page_title'];
			} elseif ( ! empty( $page['menu_title'] ) && empty( $page['page_title'] ) ) {
				$page['page_title'] = $page['menu_title'];
			}

			self::$pages[ $key ] = add_submenu_page( $page['parent_slug'], $page['page_title'], $page['menu_title'], $page['capability'], $page['menu_slug'], $page['callback'] );
			// For backward compatibility.
			$GLOBALS[ "popmake_" . $key . "_page" ] = self::$pages[ $key ];
		}

		// Add shortcut to theme editor from Appearance menu.
		add_theme_page( __( 'Popup Themes', 'popup-maker' ), __( 'Popup Themes', 'popup-maker' ), 'edit_posts', 'edit.php?post_type=popup_theme' );
	}