AJAX POST到PHP无法正常工作

I am trying to POST a JSON to PHP where it should be decoded and pushed to MySQL database.

This is my JavaScript code.

    var dictstring = "{sensorid: \""+ name +"\", x: " +pos.x +" ,y: "+pos.y+"}";
                    console.log(dictstring);

                    $.ajax({
                        type: "POST",
                        contentType: 'application/json; charset=utf-8',
                         dataType: "json",
                        url: "myfile.php",
                        data: JSON.stringify(dictstring),
                        success: function(data){
                        alert('Success');
                        },
                    error: function(e){

                    console.log(e.message);
                        }
                    });

And this is my PHP code

<?php


$jsonData = file_get_contents('php://input');

$data_back = json_decode($jsonData);
$unit = $data_back->{"sensorid"};
$x_axis = $data_back->{"x"};
$y_axis = $data_back->{"y"};


// Create connection
$con=mysqli_connect("xxxxxxx:xxxx","xxxxx","xxxx","xxxxxxxx");

// Check
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

//Insert Values
$sql_hm = "INSERT INTO xxxx(unit, x_axis, y_axis)
VALUES ('$unit', $x_axis, $y_axis)";

if ($con->query($sql_hm) === TRUE) {
    echo "values inserted successfully"; 

} else {
     echo "No data ";
}



?>

I know the PHP part works - I tried POSTing JSON data with contentType: application/json from a REST client app and it works. But when I am trying it with my Javascript code, it isnt POSTing. I can see the "dictstring" string value in console. I cannot see the "Success" alert too. Interestingly, "Success" alert is shown if I remove the dataType: "json" line. But, POSTing is still not happening as I cannot see the values in database.

What am I missing here?

var dictstring = "{sensorid: \""+ name +"\", x: " +pos.x +" ,y: "+pos.y+"}";

You have a string.

data: JSON.stringify(dictstring),

Which you then convert to a JSON representation of that string.

$data_back = json_decode($jsonData);

Which you decode to a string.

$unit = $data_back->{"sensorid"};

Which you try to treat like an object.

Start with an object, not a string:

var dictstring = { sensorid: name, x: pos.x, y: pos.y };

… you probably want a different variable name too.

dictstring is already in JSON format, you don't need JSON.stringify in your JS

Okay it might sound silly but have you made sure to state that you're using JQuery in the java script because the amount of times I've been doing ajax and i cant figure out why it wont work and its because I haven't declared the JQuery.

like this:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js">

dataType: "json" is to be used if the data returned from server is JSON

You can create an array of messages and then use echo json_encode($array); to return JSON data to your js.

Coming to your problem, from the question and the information you've supplied, I couldn't find an obvious problem, but I could suggest to use console.log(data) in your success function and see what is returned. You'll at least know if it hit your php page or returned with some error like 404 or 500.

Try $data_back['sensorid']

or

Just print_r($data_back); and see the data structure of $data_back and then try access the data accordingly.