Can't decode received JSON to array

Question

The JS

Serializing data from a FORM, stringify it and POST it as JSON with AJAX to update.php

jQuery.fn.serializeObject = function () {
  var formData = {};
  var formArray = this.serializeArray();

  for(var i = 0, n = formArray.length; i < n; ++i)
    formData[formArray[i].name] = formArray[i].value;

  return formData;
};

$(function() {
    $('form').submit(function() {
        data = $('form').serializeObject();
        alert(JSON.stringify(data));   
        $.ajax({
            type: 'POST',
            contentType: "application/json; charset=utf-8",
            url: 'inc/update.php',
            data: {json: JSON.stringify(data)},
            dataType: 'json'
        });  
    });
}); 

The update.php file where it should be decoded to an array

$str_json = file_get_contents('php://input'); //($_POST doesn't work here)
$response = json_decode($str_json, true); // decoding received JSON to array

$name = $response['name'];

$update = $pdo->prepare("UPDATE user SET name='".$name."' WHERE id='3';");
$update->execute();//the SQL works fine with String for $name

With Tamper Data addon in Firefox i checked the POSTDATA, here it is:

json=%7B%22name%22%3A%22fff%22%7D

This is like:

json={"name":"fff"}

I'm new at JS/AJAX/JSON and I cant find my mistake. So please help me.
I've searched for many hours without success.

Answer 1

Don't know what's the point of writing serializeObject function, when you can just use serializeArray.

Javascript:

$(function() {
    $('form').submit(function(e) {
        e.preventDefault(); // Stop normal submission, which is probably why your PHP code isn't working
        data = $('form').serializeArray();
        alert(JSON.stringify(data));   
        $.ajax({
            type: 'POST',
            contentType: "application/json; charset=utf-8",
            url: 'inc/update.php',
            data: {
                json: JSON.stringify(data)
            },
            dataType: 'json'
        });  
    });
    return false;
});

PHP:

$str_json = _$_POST['json'];
$response = json_decode($str_json, true); // decoding received JSON to array
$name = $response['name'];

If you use true as 2nd argument in json_decode it returns an array not object. So you need to do

$name = $response['name'];