send var into php file from javascript

it try to use Ajax in php. is write a code and at the end send a var "Id" into a php file :

xmlhttp.open("GET", "details.php?q="+Id,true);

there is no problem in the code , details.php run but details.php give me this Error : Notice: Undefined index: Id in C:\wamp64\www\adv3\advanced\frontend\web\details.php on line 2

and here it is my seconde line in details.php :

<?php
$q = $_GET['Id'];

what is the problem ? i send Id well .