db_functions.php

<?php
  define('HOST','localhost');
  define('USER','root');
  define('PASS','');
  define('DB','demo_game_level_system');

  function connect($conn = null) {
    if ($conn == null) {
      $conn = mysqli_connect(HOST,USER,PASS,DB) or die("Błąd połączenia!");
      return $conn;
    } else {
      $conn->close();
    }
  }

  function showUserInfo($user_info) {
    foreach ($user_info as $row) {
      $rows = '<tr>'
              .'<td>'.$row['id'].'</td>'
              .'<td>'.$row['level'].'</td>'
              .'<td>'.$row['xp_all'].'</td>'
              .'<td id="xp_current_db">'.$row['xp_current'].'</td>'
              .'<td id="xp_needed">'.$row['xp_needed'].'</td>'
              .'</tr>';
    }

    return $rows;
  }

  function getUserInfo($id) {
    $conn = connect();

    $sql = 'SELECT id, level, xp_all, xp_current, xp_needed FROM users WHERE id = ?';
    $stmt = $conn->prepare($sql);
    $stmt->bind_param('i', $id);
    $stmt->execute();

    $res = $stmt->get_result();
    $stmt->close();
    connect($conn);

    return $res->fetch_all(MYSQLI_ASSOC);
  }

  function setUserInfo($key, $value, $id) {
    $conn = connect();

    $sql = 'UPDATE users SET '.$key.' = ? WHERE id = ?';
    $stmt = $conn->prepare($sql);
    $stmt->bind_param('ii', $value, $id);
    $stmt->execute();

    $stmt->close();
    connect($conn);
  }
?>