<?php
$host = "localhost";
$username = "root";
$password = "";
$baza = "lab11";

$conn = @new mysqli($host, $username, $password, $baza);
if ($conn->connect_error) {
    die('Wystąpił błąd połączenia: ' . $conn->connect_errno);
}
$conn->query("set names utf8");

$sql = "INSERT INTO komentarze (id_posta, nick, tresc) VALUE (" . $_GET['id'] . ",'" . $_POST['nick'] . "','" . $_POST['tresc'] . "')";

if ($conn->query($sql) === TRUE) {
    header("Location: post.php?id=".$_GET['id']);
} else {
    echo "Błąd: " . $sql;
}

$conn->close();