IHP: How can I save multi-select html element values in database?

Question

Let's say, I have a custom form in frontView, with "txtroles" multi-select HTML element.

I submit it to my "UpdateProjectUsersAction" and would like to save it to my "project_user_roles" which have project_id (foreign id), role_name (text), userid (foreign id).

Now, I would like "UpdateProjectUsersAction" to save each multiple values added in "txtroles" to each new rows in my "project_user_roles" table