Write JSON data to text file with PHP

Question

Problem:

I have a script that send JSON data to a PHP file in this way:

var xmlhttp = new XMLHttpRequest();
xmlhttp.open("POST", "process-survey.php");
xmlhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
xmlhttp.send(JSON.stringify({uid, selected}));

The problem is that JSON data is not written to text file using the PHP function file_put_contents().

Minimal (Working) Example:

JSON as in the console log

{
  "uid":1,
  "selected":[
     {
        "questionsid":1,
        "val":"1"
     },
     {
        "questionsid":2,
        "val":"1"
     }
  ]
}

PHP

<?php
  $uid = json_decode($_POST['uid'], true);
  $answers = json_decode($_POST['selected'], true);

  $file = $_SERVER['DOCUMENT_ROOT'] . '/association/data.txt';

  // Open the file to get existing content
  $current = file_get_contents($file);

  // Append a new id to the file
  $current .= $uid . "\n";

  foreach ($answers as $item) {
    $current .= $item . "\n";
  }

  // Write the contents back to the file
  file_put_contents($file, $current);
?>

Permissions

Added the following read/write: chmod 644 data.txt

Desired output:

uid: 1
questionid: 1, val: 1
questionid: 2, val: 1

Answer 1

Your input is json, so it wont already be broken up into parts uid, selected, so the following code is taking your json and outputting your expected result (placing it in $_POST as I presume that's what you mean).

<?php
$json = '{
  "uid":1,
  "selected":[
     {
        "questionsid":1,
        "val":"1"
     },
     {
        "questionsid":2,
        "val":"1"
     }
  ]
}';

$_POST = json_decode($json, true);

$uid = $_POST['uid'];
$answers = $_POST['selected'];

$current = ''; // fgc 

// start building your expected output
$current .= "uid: $uid\n";
foreach ($answers as $item) {
    $line = '';
    foreach ($item as $key => $value) {
        $line .= "$key: $value, ";
    }
    $current .= rtrim($line, ', ')."\n";
}

https://3v4l.org/ekAUB

Result:

uid: 1
questionsid: 1, val: 1
questionsid: 2, val: 1