count button pass the current page URL with pure javascript Ajax to the php variable

I try to pass the current page URL (window.location.href) with pure javascript Ajax to the php variable$_REQUEST['url']. what am I doing wrong?

Counter.js:

'use strict';
let clicks = document.querySelectorAll('.Counter-trigger'); // IE8
let voted = localStorage.getItem('voted');
let message = document.getElementById('Counter-message');
let count = document.getElementById('Counter-count');
let i;
let url;
let post;
let xhr;

for (let i = 0; i < clicks.length; i++) {
  clicks[i].onclick = function () {
    if (voted == 'voted') {
      message.innerHTML = "Sorry, ...!!";
    } else {
      let url = window.location.href;
      let post = url; // post string
      let xhr = new XMLHttpRequest();
      xhr.open('POST', './lib/Counter/CounterReq.php', true);
      xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
      xhr.onreadystatechange = function () {
        if (xhr.readyState != 4 || xhr.status != 200) return;
        console.log(xhr.responseText);
        message.innerHTML = "Thanks!!";
        count.innerHTML = +count.textContent+1;
      };
      xhr.send(post);
      localStorage.setItem('rate', 'voted');
      voted = 'voted';
    }
  }
}

CounterReq.php

$url = $_GET['url']; // posted from page
$origin = $_SERVER['DOCUMENT_ROOT'].parse_url($url,PHP_URL_PATH);
$file = '_Counter.txt'; // counts is saved in this document
$file_path = $origin.'_Counter.txt'; // counts is saved here
$count = file_get_contents($file_path);
if ($count == null){$count = 0; echo $count;
}
$count++; // increment count by 1
$handle = fopen($file_path, "w+");
flock($handle,LOCK_EX); // LOCK_EX (2) exclusive locking for write access
fwrite($handle, $count);
flock($handle,LOCK_UN); // LOCK_EX (3) Releases a lock
fclose($handle); // close file

index.php

<?php
$path = $_SERVER['DOCUMENT_ROOT'].'/'.$_SERVER['REQUEST_URI']; // the path to the file
$file = '_Counter.txt'; // the number of vote is saved here
$file_path = $path.'_Counter.txt'; // the number of vote is saved here
if (!file_exists($file)) { // if the file does not exist, it will be created
    fopen($file, "w"); } // open file for writing only
    $count = file_get_contents($file_path); // reads entire file into a string
    if ($count == null){$count = 0;} // if the file empty, set count 0
?>
<button class="Counter-trigger">vote</button>
<span id="Counter-count">'.$count.'</span> times<br>
<div id="ClapsCounter-message"></div>
<script async src="./lib/Counter/Counter.js"></script>

console.log:

Notice: Undefined index: url in /var/www/dist/lib/Counter/CounterReq.php on line 5