Array values returns null - php

Question

I am making post request with ajax and sending data with json format.Then I am fetching json with php and decode it into an array.When I print out whole array there is no problem. I can see the key value pairs.But when I try to print out this array's one of index value it returns null.What could be the problem?Here is my ajax request
$(function() {
$("#del").click(function() {
var file_id = 32
var file_update = 1321321
var obj = {
file_id: file_id,
file_update: file_update
};
$.ajax({
type: "post",
url: "config/ajax.php",
data: {
"file_update": JSON.stringify(obj)
},
success: function(response) {
alert(response);
}
})
})
})
Here is my php code
if (isset($_POST["file_update"])) {
$file_update = json_decode($_POST["file_update"], true);
$x = $file_update[0];
var_dump($x);
var_dump($file_update);
}

Answer 1

The $file_update array is associative, therefore it doesn't have integer keys, its keys are strings.
$file_update[0] does not exist and this statement should throw an error given you have proper error reporting configured.
If you want to access specific array values use:
$file_update['file_id'];
$file_update['file_update'];
If you want to access the first element of an associative array you can use the following:
$key = array_keys($file_update)[0];
$value = $file_update[$key];