$_REQUEST var is empty from post send - php

Question

I'm doing a ajax function for wp. But i get always the response 0. I see the code of the file admin-ajax.php and see this:
if ( empty( $_REQUEST['action'] ) )
die( '0' );
This is my js function ajax.
function fnc(){
var ajax=new XMLHttpRequest();
ajax.open("POST", "<?php echo get_site_url(); ?>/wp-admin/admin-ajax.php");
ajax.onreadystatechange= function(){
if (ajax.readyState === 4) {
if (ajax.status === 200) {
alert(ajax.responseType);
alert(ajax.responseText);
} else {
alert('There was a problem with the request.');
}
}
}
ajax.send("action=some_function");
}

Answer 1

In order to have the send string be used as form data, you will probably need to add the following header:
ajax.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
Without this, PHP will not turn the raw POST data into $_POST/$_REQUEST variables.

Answer 2

$.ajax({
type:'POST',
url:"<?php echo get_site_url(); ?>/wp-admin/admin-ajax.php",
data:'', // what you want to post
success:function(data){
alert(data);
});
}
});
}
try this

Answer 3

If you want to use javascript and XMLHttpRequest this is the full way to do that :)
function ajax_post(){
// Create our XMLHttpRequest object
var ajax=new XMLHttpRequest();
// Create data to send to our PHP file
var url = "xyz.php";
var fn = document.getElementById("a").value;
var ln = document.getElementById("b").value;
var variable = fn+" hello "+ln;
hr.open("POST", url, true);
// Set content type header for sending url encoded variables
ajax.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
// Get the onreadystatechange event for the XMLHttpRequest
ajax.onreadystatechange = function() {
if(ajax.readyState == 4 && ajax.status == 200) {
var return_data = ajax.responseText;
alert(ajax.return_data);
// Send the data to PHP now... and wait for response to update the status div
ajax.send(variable); // Actually execute the request
}
}
}