I'm trying to edit a multiple select option value

Question

In the database the value are in this format 1,6,7 but i fail to retrieve them into select menu, i have two query, i pass variable rows from the first query where i have specified select query which fetch data from database by using group_reg_no and i use that variable rows in the second query so that i can get specific activity selected   <?php require_once ("db_connection.php");
if(isset($_GET['edit_id'])) {
    $edit_sql = "SELECT * FROM group_details WHERE group_reg_no = '$_GET[edit_id]'";
    $run_sql = mysqli_query($connection, $edit_sql);
    while ($rows = mysqli_fetch_assoc($run_sql)){
     $group_name = $rows['group_name'];
     $group_reg_no = $rows['group_reg_no'];
     $leader_name = $rows['leader_name'];
     $leader_contact = $rows['leader_contact'];
     $region = $rows['region'];
     $district = $rows['district'];
     $division = $rows['division'];
     $ward = $rows['ward'];
     $village = $rows['village'];
     $activity = $rows['activity'];
     $category = $rows['category'];
     $men = $rows['men'];
     $women = $rows['women'];
     $total = $rows['total'];



}
}  else  {
    $group_name = '';
     $group_reg_no = '';
     $leader_name = '';
     $leader_contact = '';
     $region = '';
     $district = '';
     $division = '';
     $ward = '';
     $village = '';
     $activity = '';
     $category = '';
     $men = '';
     $women = '';
     $total = '';

}

?>
                <label for="activity" class="col-sm-2 control-label">Shughuli za Kikundi</label>
                <div class="col-sm-3">
                     <select class="form-control" name="activity[]"   id="activity" multiple="multiple">
                     <option value="">Select Activity</option>
                         <?php
                        $sel_activity = "SELECT * FROM activity";
                        $run_activity = mysqli_query($connection, $sel_activity);
                        while($rows = mysqli_fetch_assoc($run_activity)){
                            $array =  explode(',', $rows);
                            foreach ($array as $rows) {
                            if($activity == $rows['id']){
                                $selected = 'selected';
                            } else {
                                $selected = '';
                        }
                        echo '<option value="'.$rows['id'].'" '.$selected.'>'.$rows['activity'].'</option>';
                            }
                        }
                        ?>

                     </select>
                </div>
                </div>