Třeba takto:

<?php
$servername = "localhost";
$username = "username";
$password = "heslo";
$dbname = "nazev databaze";

$conn = mysqli_connect($servername, $username, $password, $dbname);
if (!$conn) {
    die("Pripojeni k DB se nezdarilo: " . mysqli_connect_error());
}

$sql = "SQL dotaz na manipulaci s fórem";
mysqli_query($conn, $sql);