How to call PHP session from another file.php - php

Question

I'm trying to access session variables in file2.php from file1.php, but I'm getting the following error: undefined variable in file2.php.
How can I fix this?
This is file1.php:
<?php
session_start();
header("Access-Control-Allow-Origin:*");
header("Access-Control-Allow-Credentials: true");
header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE, OPTIONS");
header("Content-Type: application/json; charset=UTF-8");
header("Access-Control-Allow-Headers: Origin, Authorization, X-Requested-With, Content-Type, Accept, x-xsrf-token");
include "config.php";
$postjson = json_decode(file_get_contents('php://input'), true);
$today = date('y-m-d H:i:s');
eif(isset($postjson['aks'])=="proses_login"){
$logindata = mysqli_fetch_array(mysqli_query($mysqli, "SELECT * FROM usuario
WHERE nomeUsuario='$postjson[nomeusuario]' AND senha='$postjson[Suasenha]'"));
if(isset($logindata['nomeUsuario'])==$postjson['nomeusuario'] AND ($logindata['senha'])==$postjson['Suasenha']){
$_SESSION['U']=$postjson['nomeusuario'];
$result = json_encode(array('success'=>true, 'msgs'=>'Bem Vindo '.$_SESSION['U']));
}else{
$result = json_encode(array('success'=>false, 'msgs'=>'Nome do Usuario ou Senha incorrecta'));
}
echo $result;
}
?>
and this is file2.php:
<?php
session_start();
header("Access-Control-Allow-Origin: *");
header('Content-Type: text/html; charset=utf-8');
$host = "mysql:host=localhost;dbname=eventosonline";
$usuario = "root";
$senha = "";
$con = new PDO($host, $usuario, $senha);
$sql = $con->prepare("SELECT * FROM usuario WHERE nomeUsuario ='".$_SESSION['U']." ' ");
$sql->execute();
I tried to include file1.php but it didn't work.