shortcode - Multiple values for one variable

I have a custom plugin that was written by someone, I can't reach anymore. It has a search option, that produces a table with seminars in different cities.

Right now I can have a query with this shortcode:

[kurssuche class="course_search" place="1"]

that shows a table with all seminars in the city "1". How can I produce a table which shows seminars for two or more cities?

I'm new to this, any help is much apprechiated!

This is the code:

    if(isset($_REQUEST['search']) || $atts['place']!='')
    {
        if(isset($_REQUEST['city']) && $_REQUEST['city'])
        {
            $where[] = "city = '".$_REQUEST['city']."'";
        }
        else if($atts['place'] && $_REQUEST['city']!=='0')
        {
            $where[] = "city = '".$atts['place']."'";
        }

            $courseplanList = $wpdb->get_results($query);
        }
    }