Fetch

Method 1:

fetch('test2.php', {
    method: 'post',
    headers: {"Content-type": "application/x-www-form-urlencoded; charset=UTF-8"},
    body: 'foo=bar&lorem=ipsum' // similar GET request
  })

Method 2:

let fd = new FormData;
fd.append('start', 1);
fd.append('selected', JSON.stringify([1,2,3,4,5]))
  fetch('test2.php', {
    method: "POST",
    body: fd
  })
  .then((response)=>{
      if (response.status == 200) { // successful ajax request
        console.log('Succefull request to ' + response.url + ' // Response type = ' + response.type);
      }
      else {console.log('Error request to ' + response.url + ' //status ' + response.status + ' ' + response.statusText)} // unsuccessful ajax request
      // to read the text, php returned:
      return response.text();
      // return response.json();
    })
    .then((resp)=> {
      console.log(resp)
    })
    .catch((error)=>{
      console.log('No data returned, possible error occurred in php file.')
    });

PHP for method 2:

<?
  var_dump($_POST['selected']);
  $s = $_POST['selected'];
  var_dump(json_decode($s));
?>