Session headache

[viraj]

Well the situation is as follows:
I want to update user details and I use a session to store the user id.

Form showuserdetails saveuserdetails

In showuserdetails i put a session as follows:
$id=$POST['id'];
session_start();
$SESSION['id']=$id;
session_write_close();

In saveuserdetails i retrieve the session:
<?php
require_once 'masterconfig.php';

$funame=$POST['funame'];
$luname=$POST['luname'];
$uage=$POST['uage'];
session_start();
$id=$SESSION['id'];

$mySQLstudent = "UPDATE fullname SET Firstname='$funame', Lastname='$luname', Age='$uage' WHERE ID='$id'";
mysql_query($mySQLstudent)or die('Could not select database:' . mysql_error());

echo "User details have been modified!<p></p>";
?>

The user details are updated but I get the following message, why is it so? Can some1 plz tell me. Thanks

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at c:\wamp\www\php website\php and mysql\basic of using database - operations\savedetails.php:10) in c:\wamp\www\php website\php and mysql\basic of using database - operations\savedetails.php on line 16

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at c:\wamp\www\php website\php and mysql\basic of using database - operations\savedetails.php:10) in c:\wamp\www\php website\php and mysql\basic of using database - operations\savedetails.php on line 16
User details have been modified!

[nbop]

put session_start() at the very beginning of the page, its header info. :p

[crazychris]

and remember that if you use session_write_close() then ANY changes from that point on to the $_SESSION will not be saved. You can edit the contents, access the contents, but changes are frozen at the point you called session_write_close().
Note that sessions close themselves at the end of the script, so unless you have a good reason to manually close it, don't bother.