Problem UPDATE with AJAX

I want to update the data of a form using AJAX (jQuery 1.10.2) and the answer goes through the error.

HTML

<form name="formulario">
  <p id="identificador">9</p>
  <input id="opcion-1" type="radio" name="opcion" value="1">Opción uno
  <input id="opcion-2" type="radio" name="opcion" value="2">Opción dos
  <input id="opcion-3" type="radio" name="opcion" value="3">Opción tres
  <textarea id="texto"></textarea>
  <button onclick="update()">Update</button>
</form>

JavaScript

// Variables
var form   = document.formulario;
var id     = document.getElementById('identificador');
var radio  = document.getElementsByName('opcion');
var text   = document.getElementById('texto');


// Devolvemos la hora
function getUpdatedAt() {

  var d = new Date();
  var day = d.getDate();
  var month = d.getMonth() + 1; //Months are zero based
  var year = d.getFullYear();
  var hour = d.getHours();
  var minutes = d.getMinutes();
  var seconds = d.getSeconds();

  return year + "-" + month + "-" + day + " " + hour + ":" + minutes + ":" + seconds;

}


// Gestionamos el UPDATE
function update() {

  // Guardamos los valores que vamos a enviar
  var valor_1 = parseInt( id.textContent );
  for(var i=0; i<2; i++) { // radio.length = 2
    if (form.radio[i].checked) {
      var valor_2 = parseInt( form.radio[i].getAttribute('value') );
    }
  }
  var valor_3 = text.value;
  var valor_4 = getUpdatedAt();

  // Mandamos los datos al servidor
  $.ajax({
    url: 'classes/update.php',
    type: 'post',
    dataType: 'json',
    data: { id: valor_1, opcion: valor_2, texto: valor_3, updatedAt: valor_4 },
    success: function(response) { console.log('success'); },
    error: function(response) { console.log('error'); }
  });

}

PHP

// Nos conectamos
$db_connection = new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME, DB_PORT) or die("Error de conexión");

// cambiamos el juego de caracteres a UTF-8
$db_connection->set_charset("utf8");

$data = array();

// Hacemos el UPDATE de el input radio 'opcion' y el textarea 'texto' relacionados con el id N
$sql = "UPDATE nombre_tabla SET opciones = ".$_POST["opcion"].", textos = ".$_POST["texto"].", updated_at = ".$_POST["updatedAt"]." WHERE ids = ".$_POST["id"]."";

// pon el resultado en la variable result
$result = $db_connection -> query($sql);

$row_goal_detail = mysqli_fetch_assoc($result);

$data[] = $row_goal_detail;

echo json_encode($data); //devolvemos los datos


$db_connection->close();

The code that is described in the question is a simplified code to make it more readable. Below is the answer that paints the console of the code without simplifying:

console.log (response)

Object {
  readyState: 4,
  responseText: "<br />↵<font size='1'><table class='xdebug-error x… 11.759 alunos na Educação de joven' à la ligne 1",
  status: 200,
  statusText: "OK"}
  abort: (e)
  always: ()
  complete: ()
  done: ()
  error: ()
  fail: ()
  getAllResponseHeaders: ()
  getResponseHeader: (e)
  overrideMimeType: (e)
  pipe: ()
  progress: ()
  promise: (e)
  readyState: 4
  responseText: "<br />↵<font size='1'><table class='xdebug-error xe-warning' dir='ltr' border='1' cellspacing='0' cellpadding='1'>↵<tr><th align='left' bgcolor='#f57900' colspan="5"><span style='background-color: #cc0000; color: #fce94f; font-size: x-large;'>( ! )</span> Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, boolean given in C:\wamp\www\new-g\classes\update.php on line <i>24</i></th></tr>↵<tr><th align='left' bgcolor='#e9b96e' colspan='5'>Call Stack</th></tr>↵<tr><th align='center' bgcolor='#eeeeec'>#</th><th align='left' bgcolor='#eeeeec'>Time</th><th align='left' bgcolor='#eeeeec'>Memory</th><th align='left' bgcolor='#eeeeec'>Function</th><th align='left' bgcolor='#eeeeec'>Location</th></tr>↵<tr><td bgcolor='#eeeeec' align='center'>1</td><td bgcolor='#eeeeec' align='center'>0.0000</td><td bgcolor='#eeeeec' align='right'>144312</td><td bgcolor='#eeeeec'>{main}(  )</td><td title='C:\wamp\www\new-g\classes\update.php' bgcolor='#eeeeec'>..\update.php <b>:</b>0</td></tr>↵<tr><td bgcolor='#eeeeec' align='center'>2</td><td bgcolor='#eeeeec' align='center'>0.0010</td><td bgcolor='#eeeeec' align='right'>152720</td><td bgcolor='#eeeeec'><a href='http://www.php.net/function.mysqli-fetch-assoc' target='_new'>mysqli_fetch_assoc</a>↵(  )</td><td title='C:\wamp\www\new-g\classes\update.php ' bgcolor='#eeeeec'>..\update.php <b>:</b>24</td></tr>↵</table></font>↵[null]Erreur de syntaxe près de 'o fim de março de 2014 foram matriculados  11.759 alunos na Educação de joven' à la ligne 1"
  setRequestHeader: (e,t)
  state: ()
  status: 200
  statusCode: (e)
  statusText: "OK"
  success: ()
  then: ()
  __proto__: Object