<form method="POST">
<input type="text" name="username" placeholder="Username">
<input type="password" name="pass" placeholder="Passwort">
<input type="password" name="security" placeholder="Securitycode">
<input type="submit" name="submit" value="Einloggen">
</form>

<?php

if(isset($_POST['submit']))
{
if(isset($_POST['username'], $_POST['pass'], $_POST['security']))
  {
      if(!empty($_POST['username']) && !empty($_POST['pass']) && !empty($_POST['security']))
      {
    $hkusername = $conn->real_escape_string($_POST['username']);
    $hkpassword = HoloHashMD5($_POST['pass']);
    $hksecurity = $conn->real_escape_string($_POST['security']);


    $query = $conn->query("SELECT * FROM users WHERE username LIKE '".$hkusername."' AND password LIKE '".$hkpassword."' AND secretcode LIKE '".$hksecurity."'");
    $acpq = $query->fetch_array();

    if($query->num_rows > 0)
    {
        if($name == $acpq['username'])
        {
            $_SESSION['hksession'] = $acpq;
            header("Location: startseite.php");
        }
        else
        {
            $error = "Falsche Logindaten";
        }
    }
      }
  }
  else{
      $error = "Nicht alle Felder ausgefüllt";
      }
}

?>

<?php
if(isset($error)){
            echo $error;
        }
        ?>