posting dynamic arrays as distinct variables using java script

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Javascript

PHP

Am trying to dynamically add the fields for subjects and the grades obtained, but am getting an error "Undefined index: subject in..." when posting those variables using java script.
Could there be something am missing with my posting mechanism. Notice that in the form data am not puting id="subject" to avoid picking the id for only one subject, but I
dont seem to know how to represent this in the java script as we will see below.


form data as follows;

require_once("connection/connectPDO.php");
$sql="CALL sp_getSubjects()";

//Initiate and Call Stored Procedure Using PDO
$pdo = new PDOConfig();
$resultsSubject = $pdo->query($sql);
foreach($resultsSubject as $rowSubject)
{
?>
<select name="grades[]" id="grades" class="validate[required]">
<option value="">--Select Grade--</option>

$sql="CALL sp_grabGrades()";

//Initiate and Call Stored Procedure Using PDO
$pdo = new PDOConfig();
$resultset = $pdo->query($sql);
foreach($resultset as $row)
{

?>
<option value=""> </option>

</select>

the form looks like this

English <--select-->
Biology <--select-->
Science <--select-->

the java script code is as follows;

$(document).ready(function(){
$("#submit").click(function(){
//if invalid do nothing
if(!$("#formD").validationEngine('validate')){
return false;
}
var vgrades = $("#grades").val();
var vsubject = $("#subject").val();

$.post("sendInitialApplication.php",
{
grades : vgrades,
subject : vsubject
/*Handles response from server*/
function(response){
alert(response);
});
alert("You are here");
});
});

the PHP code "sendInitialApplication.php" is as follows

$method = $_SERVER['REQUEST_METHOD'];


function connect(){
try{
$dbConn = new PDO('mysql:host=localhost; dbname=student', 'root', 'root');
$dbConn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
return $dbConn;
}
catch(PDOException $e){
echo $e->getMessage();
}
}

/*Checks if method is HTTP POST*/
if(strtolower($method) == 'post'){

$grades = addslashes($_POST['grades']);
$subjects = addslashes($_POST['subject']);

try {

$dbHandler = connect();
$dbHandler->beginTransaction();

//Saving Various subjects with distinct grade obtained
foreach($subjects as $key => $subject)
{

$setIndexSubject = 'CALL sp_sendIndexSubject(:vSubjectID,:vGradeObtainedID)';
$stmt_subject = $dbHandler->prepare($setIndexSubject);
$stmt_subject->bindValue(':vSubjectID', $subject);
$stmt_subject->bindValue(':vGradeObtainedID', $grades[$key]);
$stmt_subject->execute();
$stmt_subject->closeCursor();
}

$dbHandler->commit();

echo "The Operation was Successful!!!!!!";

} catch (PDOException $e) {
$dbHandler->rollback();
die($e->getMessage());
}

}else{
echo "Oops! Make sure Method is POST";
}
?>