public function user_exists()
  {
    mysql_connect("localhost","root","") or die(mysql_error());
    mysql_select_db("oop") or die (mysql_error());

    $data = mysql_query("SELECT ID FROM users WHERE username =     
'{$this->username}'");

    return mysql_num_rows($data)? 1 : 0;
  }

<?php
public function user_exists(){
$dsn = 'mysql:dbname=oop;host=localhost;';
$user = 'root';
$password = '';

try {
    $pdo = new PDO($dsn, $user, $password);
      
}
catch (PDOException $e)
{
    echo 'Proba nawiazanie polaczenia zakonczyla sie niepowodzeniem:     
';
    $e->getMessage();
}

$result = $pdo->query(SELECT ID FROM users WHERE username =     
'{$this->username}');
}

return mysql_num_rows($data)? 1 : 0;