So this is driving me crazy! If the username is correct then it compares the password perfectly fine, but if the username is wrong then the comparison doesn't happen and it throws me this error. I want to compare the database value with the value entered by the user.
<?php
$nm = $_POST['nm'];
$pw = $_POST['pw'];
try{
$pdo = new PDO('mysql:host=localhost;dbname=gold-market_main', 'root', '');
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}catch(PDOException $e) {
echo "Connection failed: ".$e->getMessage();
die();
}
if($nm == null){
die("Feld darf nicht leer sein!");
} elseif(ctype_alpha($nm[0]) or ctype_digit($nm[0])){
$sql = "SELECT k_nutzername, k_passwort FROM kunden WHERE k_nutzername IN('$nm');";
$result = $pdo->query($sql);
$row = $result->fetch(PDO::FETCH_ASSOC);
if("{$row['k_nutzername']}" != $nm) {
//header("Location: login_wrongUN.html");
print("nm wrong");
} elseif("{$row['k_passwort']}" != $pw) {
//header("Location: login_wrongPW.html");
print("pw wrong");
} else {
header("Location: konto.html");
}
}else{
die("Nutzername muss mit einem buchstaben oder einer Zahl beginnen!");
}
$pdo = null;
?> 
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You can do something similar. However, it does not protect against insecure passwords a> nor is it a timed attack.
setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); }catch(PDOException $e) { echo "Connection failed: ".$e->getMessage(); die(); } if($nm == null){ die("Feld darf nicht leer sein!") } //ctype does not protect $sql = $pdo->prepare("SELECT k_nutzername, k_passwort FROM kunden WHERE k_nutzername = ?;"); $sql->bindValue(1,$nm,PDO::PARAM_STR); //bind a value to a query, called parametrized queries, most secure way against SQL injection. $sql->execute(); $row = $sql->fetch(PDO::FETCH_ASSOC); if(!$row) { // if the username not exists //header("Location: login_wrongUN.html"); print("nm wrong"); } elseif($row['k_passwort'] != $pw) { //header("Location: login_wrongPW.html"); print("pw wrong"); } else { header("Location: konto.html"); } $pdo = null; ?>