Data sent through AJAX not available in $_POST - php

Question

My Javascript code is the following:
function on(logged_user) {
alert(logged_user);
$.ajax({
url: "update_stats.php",
type: "POST",
data: logged_user
});
}
update_stats.php contains
<?php
$logged_user = $_POST["logged_user"];
?>
but I can see that $logged_user is just an empty string (I'm inserting it into a database table)

Answer 1

Your data parameter for the $.ajax call is not in the right format. From the manual:
The data option can contain either a query string of the form
key1=value1&key2=value2, or an object of the form {key1: 'value1',
key2: 'value2'}
You should change that line to:
data: { logged_user : logged_user },
or
data: 'logged_user=' + logged_user,

Answer 2

just try this:
function on(logged_user) {
alert(logged_user);
$.ajax({
type : 'POST',
url : update_stats.php,
data : logged_user,
dataType : 'json',
encode : true
});
}

Answer 3

Its not javascript , its a jquery ajax, so please include a jquery library.
and change your function like this
Syntax to pass the values like,
data: '{ "key":' + value+ ', "key":"value" }',
or
data = "key=" + value
or
data: JSON.stringify({ key: value, key: value}),
function on(logged_user) {
var dataString = "logged_user=" + logged_user
$.ajax({
type: "POST",
url: "update_stats.php",
data: dataString,
cache: false,
success: function(result) {
alert(result)
}
})
}

Answer 4

You need to pass data in key - value format to get accesible by $_POST, $_GET and $_REQUEST array variables in php.
data: {'logged_user' : data: logged_user}
You can access raw input like JSON data or text data which not in the key value format you can use file_get_contents("php://input") to access data.