将AJAX发送到PHP不显示

I have the following jquery:

$.ajax({
    type: "POST",
    url: "getchoices.php",
    dataType: 'json',
    data: JSON.stringify({ "userName": "test1", "password" : "test2" })
});

With corresponding PHP:

<?php
    $json = $_POST;
    print_r($json);
?>

However my output for getchoices.php is just Array ( )

I have implemented the solution below, with the correct output from the console, however the PHP is not displaying anything.

No need to stringify the data.Just try like this

$.ajax({
        type: "POST",
        url: "getchoices.php",
        dataType: 'json',
        data: { "userName": "test1", "password" : "test2" }
    });

And in getchoices.php

//$_POST['userName'] = test1;
//$_POST['password'] = test2;

$json = $_POST;
echo json_encode($json);

use this php function json_encode

$test = array('foo' => 'bar'); 
echo json_encode($test);