how to refresh a php variable without reloading page

1window.addEventListener('load', function()
2{
3    var xhr = null;
4
5    getXmlHttpRequestObject = function()
6    {
7        if(!xhr)
8        {               
9            // Create a new XMLHttpRequest object 
10            xhr = new XMLHttpRequest();
11        }
12        return xhr;
13    };
14
15    updateLiveData = function()
16    {
17        var now = new Date();
18        // Date string is appended as a query with live data 
19        // for not to use the cached version 
20        var url = 'livefeed.txt?' + now.getTime();
21        xhr = getXmlHttpRequestObject();
22        xhr.onreadystatechange = evenHandler;
23        // asynchronous requests
24        xhr.open("GET", url, true);
25        // Send the request over the network
26        xhr.send(null);
27    };
28
29    updateLiveData();
30
31    function evenHandler()
32    {
33        // Check response is ready or not
34        if(xhr.readyState == 4 && xhr.status == 200)
35        {
36            dataDiv = document.getElementById('liveData');
37            // Set current data text
38            dataDiv.innerHTML = xhr.responseText;
39            // Update the live data every 1 sec
40            setTimeout(updateLiveData(), 1000);
41        }
42    }
43});
44

1function data() {
2        var ret = [];
3        ret.push({
4          y: 'Today',
5          a: <?php echo $data; ?>
6        });
7      return ret;
8    }
9
10var graph = Morris.Bar({
11        element: 'graph',
12        data: data(),
13        xkey: 'y',
14        ykeys: ['a'],
15        labels: ['random label']
16    });
17
18function update() {
19      graph.setData(data());
20    }
21
22setInterval(update, 60000);
23