未捕获的ReferenceError：未定义$ _POST

I'm new in AJAX and want to send 2 IDs on an AJAX page here is my code

<a href="#" id="<?php echo $st_rows4['module_id'];?>" class="edit3">Click Here</a>

<script>
$(document).ready(function() {
    $(".edit3").click(function() {
        var moduleID    = $(this).attr('id');
        var studentID   = $_POST['studentAssignID']; //Problem is Here
        $.ajax({
            type: "POST",
            url: 'assign-assignment-ajax.php',
            data: "moduleID="+moduleID+"&studentID="+studentID,
            success: function(data)
            {
                $("#editform2").html(data);
                $("#editform2").show('slow');
            }
        });
    });
});

I'm getting module ID by clicking on a href, while I also want student ID with it, which part i'm doing wrong?

try this :

var studentID   =  "<?php echo $_POST['studentAssignID']; ?>";

Declare 'StudentAssignID' in hidden field.

<input type='hidden' class="StudentAssignID" value="<?echo $_POST['studentAssignID'];?>">

Then Use This Value in Script.

<script>
.
.
var studentID   = $('.StudentAssignID').val();
.
.
</script>