如何使用下拉框中的值填充表格？

I have three tables and two dropdown boxes. I have populated two of the tables and entered their values into the dropdown boxes. Now, I would like to choose a value from both of the dropdown boxes and put their values into the third dropdown box.

    <fieldset>
            <legend>New Cast</legend>
            <table>
            <form action="04castdb.php" method="post">
            </fieldset>
<?php

$query = "select * from dbhx_movie";
$result = mysql_query($query);
if(!$result)
{
    die("query failed");
}
    echo "<select name='movie'>";

    while($row = mysql_fetch_array($result))
    {
        echo "<option value='$row[pk]'>$row[name]</option>";
    }
        echo "</select>movie</br>";

    echo "<select name='actor'>";

$query = "select * from dbhx_actor";
$result = mysql_query($query);
if(!$result)
{
    die("query failed");
}

while($row = mysql_fetch_array($result))
{
        echo "<option value='$row[pk]'>$row[surname]</option>";
    }
    echo "</select>actor</br>";
    echo "<input type='submit' value='swag'/>";
?>
</table>
</fieldset>

This is the code I have used to populate the dropdown boxes. But when I press submit, it doesn't enter the data from the chosen values. Can anyone make some sense of it?