<div class="form-group row">
      <label class="col-md-3 form-control-label">{l s='Subject'
d='Shop.Forms.Labels'}</label>

      <div class="col-md-6">

       <select id="id_contact" name="id_contact"
onchange="showElemFromSelect('id_contact', 'desc_contact')">
        <option value="0">{l s='Choisir...'}</option>
          {foreach from=$contact.contacts item=contact_elt}
            <option value="{$contact_elt.id_contact|intval}"  {if
isset($smarty.post.id_contact) && $smarty.post.id_contact == $contact_elt.id_contact}selected="selected" {/if}>{$contact_elt.name}</option>
          {/foreach}
        </select>

       </div>

       {foreach from=$contact.contacts item=contact_elt }
  <div id="desc_contact" style="display:none;">
                    <p class="
{$contact_elt.id_contact|intval}_desc_contact_p">
                        {$contact_elt.description}
                    </p>
                    </div>
                {/foreach}

      </div>

function showElemFromSelect(select_id, elem_id)
{
var select = document.getElementById(select_id);
for (var i = 0; i < select.length; ++i)
{
    var elem = document.getElementById(elem_id + select.options[i].value);
    if (elem != null)
        toggle(elem, i == select.selectedIndex);
}
}