Multiple selections into input box - php

Question

I have a selection of countries that a user can choose from. They are displayed in a dropdown box. At the moment I can select one country and it will populate it in the input box, however i want to know if there is a way of selecting more than one and it puts them in the same box.
The data will be populated from a mysql database
Hope im making my self clear.
Here is my code so far:
JAVASCRIPT:
<script language="JavaScript" type="text/javascript">
<!--
function showSelected()
{
var selObj = document.getElementById('countries');
var txtTextObj = document.getElementById('txtText');
var selIndex = selObj.selectedIndex;
txtTextObj.value = selObj.options[selIndex].text;
}
//-->
</script>
HTML:
<form>
<p>
</form><select id="countries">
<option value="val0">Australia</option>
<option value="val1">England</option>
<option value="val2">France</option>
<option value="val3">Italy</option>
<option value="val4">South Africa</option>
</select>
<input type="button" value="Show Index" onclick="showSelected();" />
</p>
<p>
<input type="text" id="txtText" />
<br />
</p>
</form>
So what i would want is if you choose Australia, it shows Australia in the box, then go back to the dropdown and choose South Africa it puts it into the same box as: Australia, South Africa.
Thank you Guys

Answer 1

why no just append new selections to the text already in the inputbox.
txtTextObj.value += selObj.options[selIndex].text +', ';

Answer 2

Use Multiple select box and name as array:
<select id="countries" multiple="multiple" name="countries[]">
<option value="val0">Australia</option>
<option value="val1">England</option>
<option value="val2">France</option>
<option value="val3">Italy</option>
<option value="val4">South Africa</option>
</select>
Now the select will treat as array. Use javascript code to fetch the values selected and convert it to comma separated list and put in the input box.

Answer 3

<select id="countries" multiple="multiple">
<option value="val0">Australia</option>
<option value="val1">England</option>
<option value="val2">France</option>
<option value="val3">Italy</option>
<option value="val4">South Africa</option>
</select>
Use multiple="multiple" in your select box to make multiple selections.