how to send data to php file then echo a message

Question

I am trying to send data to php file then echo the word "Hello!" when i call a function in javascript, however, no message appear, i guess there is en error in the calling, can you guide me please?

Here is my code:

Javascript:

function asyncpost_deviceprint() {
var xmlhttp = false;

if (!xmlhttp && typeof XMLHttpRequest!='undefined') {
  xmlhttp = new XMLHttpRequest();
}
else if (!xmlhttp) return false;

xmlhttp.open("POST", "http://localhost/Assignment/insert.php", true);
xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

xmlhttp.send("userAgent" + userAgent()); /*  fire and forget */
return true;

}

PHP:

<?php
     echo "Hello!";
?> 

Answer 1

echo "Hello!";

won't display any message because in Ajax request this function sends a respond to Javascript. If you want to display sth on the screen with PHP instead of Ajax you should use:

window.location.href="path to your php site"

it will redirect you to php file and display Hello!

Answer 2

xmlhttp.onreadystatechange = function () {
    if (xmlhttp.readyState === 4) {
        if (xmlhttp.status === 200) {
            document.body.innerHTML += xmlhttp.responseText;
        }
    }
};

Add this before xmlhttp.send

It will literally just stick the php echo text after the last thing in the document.