How to store an array or string separated by commas?

Question

PHP

Friends, hello everyone!
Tell me, how can I achieve the storage of a list of values ​​​​in k2? In short, there is a parent field, now one value is written there, but several must be written separated by commas.
Values ​​on the form are selected normally, but only one material is saved whose value is greater.

<div class="paramValue">
              <select class="form-control" name="parent" id="parent" multiple="multiple" style="width:300px;">
                     <option value="0">Нет</option>
                            <?php foreach ($this->tags as $value): ?>
                                   <option value="<?php echo $value->id; ?>" <?php echo $this->row->parent == $value->id ? 'selected' : ''; ?>>
                                   <?php echo $value->name; ?>
                     </option>
                            <?php endforeach; ?>
              </select>
       </div>

Storage cell varchar 64.