Untitled

<form method="post" action="register.php" name="registerform">
 <input type="submit" name="register" value="registrovat" id="button8" class=I don't know what /><br><br>
</form>

#button8{
    visibility: hidden;
    font-family: century gothic;
    color: #0099FF;
    border: 2px solid #0099FF;
    background: transparent;
}
#button8.visible{
    visibility: visible;}

<!DOCTYPE html>
<html>
<head>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
    <title>Panákovač 4.0</title>
    <link rel="stylesheet" type="text/css" href="mystyle.css">
</head>


</head>
<body>


</body>
</html>

<?php
// show potential errors / feedback (from registration object)
if (isset($registration)) {
    if ($registration->errors) {
        foreach ($registration->errors as $error) {
            echo $error;
        }
    }
    if ($registration->messages) {
        foreach ($registration->messages as $message) {
            echo $message;
        }
    }
}

?>

<!-- register form -->
<form method="post" action="register.php" name="registerform">
    <input id="login_input_username" class="login_input" type="text" pattern="[a-zA-Z0-9]{2,64}" name="user_name" placeholder="uživatelské jméno" required /><br>
    <input id="login_input_password_new" class="login_input" type="password" name="user_password_new" pattern=".{6,}" placeholder="heslo (min. 6 znaků)" required autocomplete="off" /><br>
    <input id="login_input_password_repeat" class="login_input" type="password" name="user_password_repeat" pattern=".{6,}" placeholder="zopakujte heslo" required autocomplete="off" /><br>
    <input id="login_input_firstname" class="login_input" type="text" name="user_firstname" placeholder="jméno" required /><br>
    <input id="login_input_lastname" class="login_input" type="text" name="user_lastname" placeholder="příjmení" required /><br>
    <input id="login_input_email" class="login_input" type="email" name="user_email" placeholder="e-mail" required /><br>
    <input id="login_input_company" class="login_input" type="text" name="user_company" placeholder="firma" required /><br>
    <input type="submit" name="register" value="registrovat" id="button8" class= /><br><br>
</form>

<form method="post" action="register.php" name="registerform">
    <input id="login_input_code" class="login_input" type="text" name="user_code" placeholder="ověřovací kód" required /><br>
    <input type="submit"  name="kod" value="ověřit kód"/><br>
</form>

<?php
if(isset($_POST['kod'])){
            $servername = "localhost";
            $username = "root";
            $password = "";
            $dbname = "login";


            // Create connection
            $conn = new mysqli($servername, $username, $password, $dbname);
            // Check connection
            if ($conn->connect_error) {
                die("Connection failed: " . $conn->connect_error);
            }
$user_code = $_POST['user_code'];
$sql = "SELECT `used`,`code` FROM `regcode` WHERE `code` LIKE '$user_code' ";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {

        echo "kód: " . $row["code"]. " - použito: " . $row["used"]. $zob . "<br>";
        $pouzito = $row["used"];
MY CONDION FOR CLASSES
    }
} else {
    echo "chybný kód" . $user_code;
}
            $conn->close();
    }

?>

<form method="post" action="register.php" name="registerform">
  <input type="submit" name="register" value="registrovat" id="button8"
     class="<?php if(condition) { echo 'classname'; } ?>" />
</form>

<input class="<?= ($condition) ? 'classname' : ''"> ?>

echo '<input type="submit" name="register" value="registrovat" id="button8"
 class="class1">';

 echo '<input type="submit" name="register" value="registrovat" id="button8"
 class="class2">';

$class = "class1";
?>
input type="submit" name="register" value="registrovat" id="button8"
 class="<?php echo $class; ?>">
<?php

$class = "class2";
?>
input type="submit" name="register" value="registrovat" id="button8"
 class="<?php echo $class; ?>">
<?php

<?php $classname = condition ? 'class8' : 'defaultClass'; ?>
<form method="post" action="register.php" name="registerform">
        <input type="submit" name="register" value="registrovat" id="button8" class="<?php echo $classname; ?>" /><br><br>
    </form>

<form method="post" action="register.php" name="registerform">
      <input type="submit" name="register" value="registrovat" id="button8" class"<?= $condition ? 'classvisible' : 'classhidden'"?> />
    </form>

...//some condition
If ($row["used"] == 0){
        $myclass ="zobrazeno";
        }
...

<input type="submit" name="register" value="registrovat" id="button8" class=<?php echo $myclass; ?> /><br><br>

#button8{
    visibility: hidden;
    font-family: century gothic;
    color: #0099FF;
    border: 2px solid #0099FF;
    background: transparent;

#button8.zobrazeno{
    visibility: visible;