与php无法在子目录中工作的ajax连接

Hello I have a javascript file in the directory

root/ assets/ js / a.js

and a php file at the directory

root/php/foo.php

At the a.js file I am trying to use ajax in order to connect with the foo.php . If the a foo.php is located at the root directory a.k.a

root/foo.php 

the function works fine but when i put the file in the directory above nothing works. Here is the function associated with the connection

if (window.XMLHttpRequest) {
// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    } else { // code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
           alert(xmlhttp.responseText);
        }
    }

    xmlhttp.open("GET","../../studentSignupPHP/test.php?q=" + str, true);

The relative path at the

 xmlhttp.open(..)

seems fine to me,so I cannot understand what I am doing wrong. Can you help me?

foo.php

<?php echo "test"; ?>