Inserting multiple selections from list/menu

I also want such a solution where I can fill in multiple selections in a select list and save them comma-separated.

Thanks to this post I have managed to do this on the insert page. But I do not understand how to create this on the update and results / detail pages.

I want them to appear in the same way on the update page (as a populated multiple select list). And on the details page, I want it to appear as a list.

Grateful for help.

I attach the code from the insert page:

<select name="avsettfor_id[]" required multiple class="form-control" id="avsettfor_id" tabindex="7" rel="<?php echo((isset($_GET["invalid"])?ValidatedField("laromedelupdate","avsettfor_id"):"")); ?>">
<option value="" <?php if (!(strcmp("", (isset($_GET["invalid"])?ValidatedField("laromedelinsert","avsettfor_id"):"")))) {echo "selected=\"selected\"";} ?>>Välj avsett för</option>
<?php
while(!$WADAMenuavsettfor_id->atEnd()) { //dyn select
?>
<option value="<?php echo($WADAMenuavsettfor_id->getColumnVal("id")); ?>"<?php if (!(strcmp($WADAMenuavsettfor_id->getColumnVal("id"), (isset($_GET["invalid"])?ValidatedField("laromedelinsert","avsettfor_id"):"")))) {echo "selected=\"selected\"";} ?>><?php echo($WADAMenuavsettfor_id->getColumnVal("text")); ?></option>
<?php
$WADAMenuavsettfor_id->moveNext();
} //dyn select
$WADAMenuavsettfor_id->moveFirst();
?>
</select>