post-array-to-php.php

<?php

if( !empty( $_POST ) ) {
  die( "data: " . $_POST["msg"] );
}

?>
<form method="post">
  <button type="button">Go</button>
</form>
<script>
document.querySelector("button").addEventListener("click", function() {
  var req = new XMLHttpRequest();
  req.open("POST", window.location.href, true)
  req.onreadystatechange = function() {
      if(req.readyState == 4) alert(req.responseText)
  }
  req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
  req.send("msg=" + JSON.stringify([1,2,"hi"]))
})
</script>