如何在ajax中发送POST数据？

I need to write a script that receives and parses a JSON array within the POST array. In order to do this, I'm first trying to just send any old JSON data to my script so I have something to work with.

My receiving script is in PHP (but alternately could be done in Javascript.) Right now, I'm just serializing the POST array and writing it to a text file to make sure something is coming in. What it's writing is an empty array, though.

I'm trying to send the data using an ajax request. This is what I have at the moment:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script>
    <script>
        $(document).ready(function() {  
        var jsondata = JSON.stringify({
            val1:"this",
            val2:"that"
        });  

        $.ajax({
            url: "http://api.mydomain.com/test/index.php",
            method: "POST",        
            data: {json: jsondata},
            contentType: "application/json",
            success: function(data){alert(JSON.stringify(data));},
            error: function(errMsg) {
                alert(JSON.stringify(errMsg));
            }
        });
    });
    </script>
</head>
<body>  
</body>
</html>

I've tried lots of variations on this, too, including

not stringifying jsondata
changing data to this: data: jsondata
using type: instead of method:in the request
including datatype: "json" in the request

and some other variations I can't even remember at this point. Am I missing something simple? Or is there an easier way to accomplish this?

EDIT: adding my index.php file

if (isset($_POST)){
    // This line is commented out because it breaks it.
    //$jspost = json_decode($_POST['json']);
    $jsser = serialize($_POST);
    echo "I'm here.";
    // Write to text file
    $myfile = "response.txt";
    $fh = fopen($myfile, 'w') or die("can't open file");
    $now = date("n/j/y g:i:s a");
    fwrite($fh, $now."
");
    fwrite($fh, "I received a POST.
");
    fwrite($fh, $jsser);
    fwrite($fh, "

");
    fclose($fh);
}

try this

var jsondata = {
        "val1":"this",
        "val2":"that"
    };  

    $.ajax({
        url: "http://api.mydomain.com/test/index.php",
        method: "POST",        
        data: jsondata,
        contentType: "json",
        success: function(data){
           //your success function
        },
        error: function(errMsg) {
            //your error function
        }
    });

JS
Send a JSON String

    $(document).ready(function () {
        var obj = {
            val1: "this",
            val2: "that"
        };
        obj.val3 = 'these';
        obj['val4'] = 'those';
        $.ajax({
            type: "POST",
            url: "service.php",
            data: {
                 json: JSON.stringify(obj)
            },
            success: function (response) {
                //service.php response
                console.log(response);
            }
        });
    });

service.php
Decode and work the received object

$json = filter_input(INPUT_POST, 'json');
$decoded_json = json_decode($json);
$val1 = $decoded_json->val1;

var_dump($decoded_json, $val1);

Viceversa, if you want to send a JSON from php and decode it into JS

PHP

$responseArray = array();
$responseArray['key1'] = 'value1';
$responseArray['key2'] = 'value2';

echo json_encode($responseArray);

success: function (response) {
    var decodedJson = JSON.parse(response);
    console.log(decodedJson['key1']);
}