JavaScript-to-PHP JSON Integration

I've

function Display()
{
listIask = document.getElementsByClassName("iask");

var data = [];

for(i=0;i<listIask.length;i++)
{
data.push(
listIask[i].value
)
}

var http = new XMLHttpRequest();
var url = "php/book.php?cap=1&part=1";

http.open('POST', url, true);

http.setRequestHeader('Content-type', 'application/json');

http.onreadystatechange = function() {
if(http.readyState == 4 && http.status == 200) {
array_answer_correct = JSON.parse(http.response);

alert(array_answer_correct);
}
}
http.send(JSON.stringify(data));
}
//in php
I ve

if( isset( $_GET['cap'] ) ) {
display_php();
}
function display_php()
{
print_r($_POST["Array"]);
}

it say not recognize key data.

Why?