define("dbserver", "localhost");
define("dbuser", "user");
define("dbpass", "");
define("dbname", "user");


$db = new PDO(
"mysql:host=" .dbserver. ";dbname=" .dbname,dbuser,
array(
PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8",
PDO::MYSQL_ATTR_INIT_COMMAND => "SET CHARACTER SET utf8"
) );

session_start();
include "DBjoin.php";
if(isset($_POST["email"])) {
  $_SESSION['email'] = $_POST["email"];
  }
if(isset($_POST["nick"])) {
  $_SESSION['nick'] = $_POST["nick"];
}
if(isset($_POST["pass"])) {
  $_SESSION['pass'] = $_POST["pass"];
  $_SESSION['pass'] = base64_encode($_SESSION['pass']);
}
$sthandler = $db->prepare("SELECT Email FROM Registrace WHERE Email =     :email");
$sthandler->bindParam(':email', $_SESSION['email']);
$sthandler->execute();
if(filter_var($_SESSION['email'], FILTER_VALIDATE_EMAIL)) {
if($sthandler->rowCount() > 0){
      echo "Email is used";}

$db = new PDO(
"mysql:host=" .dbserver. ";dbname=" .dbname,dbuser,dbpass,
array(
PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8",
PDO::MYSQL_ATTR_INIT_COMMAND => "SET CHARACTER SET utf8"
) );

$servername = "localhost";
$dbname = "user";

define("dbuser", "user");
define("dbpass", "");

$dsn = "mysql:host=$servername;dbname=$dbname";

$username = dbuser; // variable equals constant
$password = dbpass; // variable equals constant

$options = array(
PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8",
PDO::MYSQL_ATTR_INIT_COMMAND => "SET CHARACTER SET utf8"
);

$db = new PDO($dsn, $username, $password, $options);

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);

// rest of your code