PHP - Create default default object from empty value - php

Question

This is my code: I want to update the User information and risk factors.
public function getUserById($id)
{
$user = $this->_xml->xpath('//user[#id="' . $id . '"]');
return $user[0];
}
public function updateUser($post)
{
$user = $this->_xml->xpath('//user[#id="' . $post['id'] . '"]');
$user[0]->name= $post["name"];
$user[0]->date_of_birth= $post["date_of_birth"];
$user[0]->sex= $post["sex"];
$user[0]->age= $post["age"];
$this->_xml->asXML($this->path);
}
public function updateFactors($post)
{
$user = $this->_xml->xpath('//user[#id="' . $post['id'] . '"]');
//$user=new stdClass();
$user[0]->alcohol= $post["alcohol"]; // error here!
$user[0]->percentage= $post["percentage"];
$this->_xml->asXML($this->path);
}
if ($param == "edit") {
$id = $arr[1];
$user = $process->getUserById($id);
include 'myuser2.php';
if ($param == "update") {
$post = $_POST;
$process->updateUser($post);
include 'mylistar.php';
$process->listUsers();
}
if ($param == "editfactors") {
$id = $arr[1];
$user = $process->getUserById($id);
include'factors.php';
}
if($param == "updatefactors"){
$post = $_POST;
$process->updateFactors($post);
include 'mylistar.php';
$process->listUsers();
}
So, the "update", "edit", the updateUser function is working... But the Factors part is not! The code is pretty much the same and I don´t know what I'm doing wrong... Seems logic to me.
Error : Creating default object from empty value (in line
$user[0]->alcohol= $post["alcohol"]; )
I've searched and tried $user=new stdClass(); but it doesn't work:
Error: Cannot use object of type stdClass as array
Can you help? :/