<?php
$name = $_POST['mcName'];

$servername = "localhost";
$username   = "benutzername";
$password   = "passwort";
$database   = 'datenbankname';

$db = @new mysqli($servername, $username, $password, $database);


if (mysqli_connect_errno()) {
    die ('Konnte keine Verbindung zur Datenbank aufbauen: '.mysqli_connect_error().'('.mysqli_connect_errno().')');
}

$name = trim($_POST['mcName']);
$name = $db->real_escape_string($name);

$db->query("UPDATE mc_prem SET balance = '1500' WHERE username = '".$name."'");