Passing data from AJAX to PHP with POST method without using <form> element

I'm having a "Undefined index" while passing values from ajax to php without using a form. Here are my codes: function sendDatasToPhp() { var id = <? php echo json_encode($id); ?> ; var otherId = <? php echo json_encode($_SESSION['id']); ?> ; $.ajax({ method: "POST", url: "send_data_to_php.php", data: "id=" + id + "&otherId=" + otherId, cache: false, success: function() { alert("Datas are being transfered"); } }) } <div> <label class="send_datas" onclick="sendDatasToPhp();">Send datas to php</label> </div> Assuming that $id is already initialized and $_SESSION['id'] has already a value. The problem is that the two indices are undefined once transferred to the php file. How can I fix these?