问使用php mysql将选定下拉列表值的主键作为外键插入到另一个表中

EN

<form id="country" name="form1" method="post" action="datatodb.php">
<table border="1">
<tr>
<td>
Select Country :<select name="countryname">
<?php
include 'dblogin.php';
// Get records from database (table "name_list").
$list=mysql_query("select * from _countries");
// Show records by while loop.
while($row_list=mysql_fetch_assoc($list)){
?>
<option value="<?php echo $row_list['countryid']; ?>"><?php echo $row_list['countryname']; ?></option>
<?php
// End while loop.
}
?>
</select> </td></tr>
<tr><td>
Add Location :   
<input type="text" name="locationname" value="" maxlength="40"/></td></tr>
<tr><td align="center">
<input type="submit" name="Submit" value="Add" size="20"/></td></tr></table>
</form>

<?php
include 'dblogin.php';

$countryid=$_POST['countryname']; 
$locationname=$_POST['locationname'];

$countryid = stripslashes($countryid); 
$locationname = stripslashes($locationname);

$countryid = mysql_real_escape_string($countryid);
$locationname = mysql_real_escape_string($locationname);

$sql="insert into _location (locationid, locationname, countryid) values ('NULL','".$locationname."','".$locationname."')";
$result=mysql_query($sql);
?>