var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    var myObj = JSON.parse(this.responseText);
    document.getElementById("demo").innerHTML = myObj.name;
  }
};
xmlhttp.open("GET", "demo_file.php", true);
xmlhttp.send();

$myArr = array("name"=>"John", "age"=>30, "city"=>"New York");

$myJSON = json_encode($myArr);

echo $myJSON;