Empty $_POST Array parsing JS Object to PHP via JSON - php

Question

I assume some of you might be already rolling their eyes since my topic has been dealt with so often in this forum. However, I haven't found any solution yet in this forum.
I want to parse a JS Object to a receiving PHP Site via JSON. I already read numerous times about the right parameters for the XMLHttpRequest-Header and its impact on the PHP part. I tried the very same solutions given in several other forums, but IT SIMPLY DOESN'T WORK for me. I've been working on this whole issue for four month now. I really need some advice.
Here is my JSON encoding JS Script:
function saveToDB(knotItems) {
var txtobj = knotItems;
var json = JSON.stringify(txtobj);
var url = "http://localhost/projektplaner/tools/DBConnection/writeFileToDB.php";
rq = new XMLHttpRequest();
rq.open("post", url, true);
rq.setRequestHeader('Content-type', 'application/x-www-form-urlencoded; charset=UTF-8');
rq.send("project=" + json);
rq.onreadystatechange = window.open(url);
}
This is the JS-generated object array knotItems after being stringified by JSON:
[{
"rank": 1,
"name": "Baugrube",
"faz_obj": "2016-05-30T17:52:16.402Z",
"fazInDays": null,
"faz_string": "19.5.2016",
"faz_timestamp": 1463680336402,
"d": "2",
"parallelTask": "seriell",
"fez": ["2016-05-21T17:52:16.402Z", "21.5.2016", "NaN2"],
"fez_dateObj": "2016-05-21T17:52:16.402Z",
"fez_string": "21.5.2016"
}, {
"rank": 2,
"name": "Kellerwände",
"faz_obj": "2016-05-30T17:52:16.402Z",
"fazInDays": null,
"faz_string": "21.5.2016",
"faz_timestamp": 1463853136402,
"d": "4",
"parallelTask": "seriell",
"fez": ["2016-05-25T17:52:16.402Z", "25.5.2016", "NaN4"],
"fez_dateObj": "2016-05-25T17:52:16.402Z",
"fez_string": "25.5.2016"
}, {
"rank": 3,
"name": "Kellerdecke",
"faz_obj": "2016-05-30T17:52:16.402Z",
"fazInDays": null,
"faz_string": "25.5.2016",
"faz_timestamp": 1464198736402,
"d": "5",
"parallelTask": "seriell",
"fez": ["2016-05-30T17:52:16.402Z", "30.5.2016", "NaN5"],
"fez_dateObj": "2016-05-30T17:52:16.402Z",
"fez_string": "30.5.2016"
}]
The reason I post the variable content is to show that all object keys are in double quotes, as required by PHP's json_decode.
This is my PHP receiving script:
if(isset($_POST['project']))
{
echo json_decode($_POST['project'],true);
}
else
{
echo "Keine Daten"; // No Data
}
This is the parameter payload that can be seen via Firebug analysis:
project:"[{"rank":1,"name":"Baugrube","faz_obj":"2016-05-30T17:52:16.402Z","fazInDays":null,"faz_string":"19.5.2016","faz_timestamp":1463680336402,"d":"2","parallelTask":"seriell","fez":["2016-05-21T17:52:16.402Z","21.5.2016","NaN2"],"fez_dateObj":"2016-05-21T17:52:16.402Z","fez_string":"21.5.2016"},{"rank":2,"name":"Kellerwände","faz_obj":"2016-05-30T17:52:16.402Z","fazInDays":null,"faz_string":"21.5.2016","faz_timestamp":1463853136402,"d":"4","parallelTask":"seriell","fez":["2016-05-25T17:52:16.402Z","25.5.2016","NaN4"],"fez_dateObj":"2016-05-25T17:52:16.402Z","fez_string":"25.5.2016"},{"rank":3,"name":"Kellerdecke","faz_obj":"2016-05-30T17:52:16.402Z","fazInDays":null,"faz_string":"25.5.2016","faz_timestamp":1464198736402,"d":"5","parallelTask":"seriell","fez":["2016-05-30T17:52:16.402Z","30.5.2016","NaN5"],"fez_dateObj":"2016-05-30T17:52:16.402Z","fez_string":"30.5.2016"}]"
HTTP Status Code is always 200 (OK).
And this is what I get from the PHP File.
Notice: Undefined index: project in C:\xampp\htdocs\projektplaner\tools\DBConnection\writeFileToDB.php on line 7
Keine Daten
I don't want to sound melodramatic or anything, but I'm about to go ape. I just can't see what the hell is wrong. Could it be a Server configuration issue?
I really, really appreciate your help.
Thank you very much in advance.