<select name="Pridej_kategorie" class="pridat_produkt_povinny_select">
     <?php
      $kategorie = MySQL_Query("SELECT * FROM Kategorie",$pripojeni);
        while($kat = MySQL_Fetch_Object($kategorie))
        {
          echo "<option value='".$kat->ID_kategorie."' selected>".$kat->Nazev_kategorie."</option>";
        }
     ?>
   </select>