Problem of XMLHttprequest, reponseText still empty - php

Question

I create articles on an administration and on each article i have a button to like it, at the onclick, the javascript call the php code in an other file to fill the database, and the database should resend the number of like, but the echo on the processing file doesn't fill the reponseText of the XMLHttprequest and I don't know why and that's I'am trying to solve and understand.
I succeeded to do it but I don't want to refresh the page at each like, so i use XMLHttpRequest to display correctly the likes whithout refreshing.
I followed serevals tutoriels, but that doesn't work well.I had to miss something.
First I have a file : index.php with my javascript and php code:
<ul class="ulp">
<li class="lip">
<button class="likebutton" onclick="like('<?= $ipclient ?>','<?= $event->getID()?>')" style="color:#5B5A57">
<i id="coeur<?= $event->getID()?>" class="<?= $classlike ?>">
<span id="nblike<?= $event->getID()?>">
<?= $tableLikes->countLike($event->getID()) ?>
</span>
</i>
</button>
</li>
</ul>
<script>
function like(ipclient,eventid)
{
var xhttp;
xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if(this.readyState == 4 && this.status == 200)
{
document.getElementById("nblike" +eventid).innerHTML = this.responseText;
};
xhttp.open("GET","event/" + ipclient + "-" + eventid, true);
xhttp.send();
}
</script>
When I am cliking on this button that send correctly the request.
I have a second file that processes the data:
<?php
use App\Model\Likes;
use App\Table\LikesTable;
use App\Connection;
$pdo = Connection::getPDO();
$like = new Likes();
$tableLike = new LikesTable($pdo);
$ip = $params['ip'];
$id_event = $params['id'];
if($tableLike->likeIp($ip,$id_event) === true){
$like->setId_article($id_event);
$like->setIp($ip);
$tableLike->newLike($like);
}else if($tableLike->likeIp($ip,$id_event) === false){
$tableLike->dislike($ip,$id_event);
}
echo $tableLike->countLike($id_event);
I would like to have the content of the last echo in the reponseText of my object XMLHttpRequest.
How can I do ?
Thanks you in advance .