php mysql WHERE clause Syntax error

[Vivekneo]

Hi, iam trying to insert values in the child table, with condition that both pk and fk should match,
pk = stud_id and fk dstud_id.
here is what iam trying

include('config.php'); 
if (isset($_GET['stud_id']) ) { 
$id = (int) $_GET['stud_id']; 
if (isset($_POST['submitted'])) { 

foreach($_POST AS $key => $value) { $_POST[$key] = mysql_real_escape_string($value); } 

$sql = "INSERT INTO `cjet`.`student_don_reg_history` (`stud_full_name`,`course_place`,`course_year`,
`course_name`,`stud_reg_no`,`don_amt`,`dd_no`) 
VALUES('{$_POST['stud_full_name']}','{$_POST['place']}',
'{$_POST['year']}','{$_POST['course']}','{$_POST['stud_reg']}',
'{$_POST['don_amt']}','{$_POST['dd_no']}')WHERE `dstud_id` = '$id' ";

mysql_query($sql) or die(mysql_error());

[Markus]

INSERTs do not have a WHERE clause.

[kovik]

If the ID is already known but you aren't sure if the record exists, you may wish to use REPLACE INTO instead of INSERT INTO.

[Max Burke]

Many Thanks Markus, your comment just ended an hour of mepulling my hair out! :)