Ajax post returning empty $_POST - php

Question

Okay I think I am going mad because I have done this a million times before and now I can't make it work. I am doing an ajax post to a PHP script with some simple JSON and then returning the JSON from my PHP, however it is currently showing $_POST as an empty array.
Here is my js:
$.ajax({
type: "POST",
url: "/account/book-promo.php",
data: '{"firstName":"Peter" , "lastName":"Jones"}',
success: function(response) {
console.log(response);
}
});
And my PHP:
<?php
var_dump($_POST);
exit;
Firebug shows that my request is using POST as it is supposed to and my data is being sent as JSON yet I am getting a response of:
array(0) {
}
The only thing I can think is that there is some kind of server settings that are preventing this from working, however I cant think why there would be. Maybe I have missed a bracket or something, it is driving me mad!
Any and all suggestions welcome!

Answer 1

send it like if you want to send it as json.
data: { data : '{"firstName":"Peter" , "lastName":"Jones"}' },
and if you want to send it as POST just remove quotes '
data: {"firstName":"Peter" , "lastName":"Jones"},

Answer 2

You have to do this:
$.ajax({
type: "POST",
dataType: "json", // <---------------its required if response is json
url: "/account/book-promo.php",
data: {"firstName":"Peter" , "lastName":"Jones"}, //<---instead of string send the object this way
success: function(response) {
console.log(response);
}
});
You said in your post retuning the JSON from my PHP:
so you need to use dataType:"json" and the data you are sending to your php should be sent as object (which is usually to be pair of key and values separated by : like {key:value}) as of your code you are sending a string.