Submitting JSON data via JQuery ajax.post to PHP

Where you went wrong in your code in the first code is that you must have used this:

var_dump(json_decode(file_get_contents("php://input"))); //and not $_POST['data']

Quoting from PHP Manual

php://input is a read-only stream that allows you to read raw data from the request body.

Since in your case, you are submitting a JSON in the body, you have to read it from this stream. Usual method of $_POST['field_name'] wont work, because the post body is not in an URLencoded format.

In the second part, you must have used this:

contentType: "application/json; charset=utf-8",
url: "ajax/selectSingle.php?m=getAbsence",
data: JSON.stringify({'Absence' : JSON.stringify(this)}),

UPDATE:

When request has a content type application/json, PHP wont parse the request and give you the JSON object in $_POST, you must parse it yourself from the raw HTTP body. The JSON string is retrieved using file_get_contents("php://input");.

If you must get that using $_POSTyou would make it:

data: {"data":JSON.stringify({'Absence' : JSON.stringify(this)})},

And then in PHP do:

$json = json_decode($_POST['data']);