Code:

if (!$_POST["prazan"])
{
        ?>
        <form method="post" action="">
                       ...............
                       ...............
        Sredstvo:<select type="sredstvo">
                  <option value="bus" selected="selected">bus</option>
                  <option value="train">train</option>
                  <option value="airplane">airplane</option>
                  </select>      <br>
        <input type="submit" name="prazan" value="Sacuvaj">
        </form>
        <?php
}
else
{
 include "connectToDeparturesMYSQL.php";   
//u if hocu da ispitam "sredstvo" i da otvorim jednu od 3 tabele
}

Code:

        $sql="INSERT INTO '$sredstvo' (city, time, ampm) VALUES ('$_POST[city]', '$_POST[time]', '$_POST[ampm]')";