<script type="text/javascript" LANGUAGE="JavaScript">

function przetwarzaj_dane ()
{
  var brakuje_danych=false;
  var formularz=document.forms[0];
  var napis="";
  if (formularz.company.value == ""){
    napis += "imie\n"
    brakuje_danych=true;
  }
  if (formularz.address_2_field.value == ""){
    napis += "nazwisko\n"
    brakuje_danych=true;
  }
  if (formularz.extra_field_1.value == ""){
    napis += "kraj\n"
    brakuje_danych=true;
  }
  if (formularz.extra_field_2.value == ""){
    napis += "telefon\n"
    brakuje_danych=true;
  }
  if (!brakuje_danych)
    formularz.submit();
  else{
    alert ("Nie wypełniłe¶ następuj±cych pól:\n" + napis);
}

</script>

<form action="<?php echo SECUREURL ?>index.php" method="post" name="adminForm" >
....
   <br/><br/><div id="city_div" style="float:left;width:40%;text-align:right;vertical-align:bottom;font-weight: bold;font-size: 10px;padding-right: 5px;"><label for="city_field"><?php echo $VM_LANG->_PHPSHOP_USER_FORM_CITY; ?></label></div>

      <div style="float:left;width:50%;">
<input type="text" id="city_field" name="extra_field_1" size="30" class="inputbox" value="<?php echo $db->f('extra_field_1');?>"/>
</div>
                  <br/><br/><div id="postc_div" style="float:left;width:40%;text-align:right;vertical-align:bottom;font-weight: bold;font-size: 10px;padding-right: 5px;"><label for="postc_field"><?php echo $VM_LANG->_PHPSHOP_USER_FORM_ZIP; ?></label></div>

      <div style="float:left;width:50%;">
<input type="text" id="postc_field" name="extra_field_2" size="30" class="inputbox" value="<?php echo $db->f('extra_field_2');?>"/>
</div>
...

<?php
                if( @VM_ONCHECKOUT_SHOW_LEGALINFO == '1' ) {
                    $link =  sefRelToAbs('index2.php?option=com_content&task=view&id='.VM_ONCHECKOUT_LEGALINFO_LINK );
                    $jslink = "window.open('$link', 'win2', 'status=no,toolbar=no,scrollbars=yes,titlebar=no,menubar=no,resizable=yes,wid
h=640,height=480,directories=no,location=no'); return false;";
                       if( @VM_ONCHECKOUT_LEGALINFO_SHORTTEXT=='' || !defined('VM_ONCHECKOUT_LEGALINFO_SHORTTEXT')) {
                        $text = $VM_LANG->_VM_LEGALINFO_SHORTTEXT;
                    } else {
                        $text = VM_ONCHECKOUT_LEGALINFO_SHORTTEXT;
                    }
                    ?>
                    <div class="legalinfo"><?php
                        echo sprintf( $text, $link, $jslink );
                        ?>
                    </div><br />
                    <?php
                   }
                    ?>
                <div align="center">
                <input type="submit" onclick="return( submit_order( this.form ) );" class="basket_input_buttons" name="submit" value="<?php echo $VM_LANG->_PHPSHOP_ORDER_CONFIRM_MNU ?>" />
                </div>
            <?php
            }
            elseif ($checkout_this_step != CHECK_OUT_GET_FINAL_CONFIRMATION) { ?>
                <div align="center">
                <input type="submit"  onClick="przetwarzaj_dane()" class="button" name="submit" value="<?php echo $VM_LANG->_PHPSHOP_CHECKOUT_NEXT;?> >>" />
                </div>
            <?php
            } ?>
            </td>
        </tr>
    </table>
</form>