No data supplied for parameters in MySQLi prepared statement

Question

I've been reworking my website from unprotected MySQL queries to mysqli prepared statements and it all went well until I got this: No data supplied for parameters in prepared statement.

if(empty($err)) {
    $pSETQuery  = NULL;
    if(!empty($_POST['password'])) {
        $pSETQuery .= ", password = ?";
    }
    if($session->isSuperuser()) {
        $pSETQuery .= ", usertype = ?";
    }
    if(!($stmt = $database->prepare("UPDATE user SET username = ?, email = ? $pSETQuery WHERE UserId = ?"))) {
        $err[] = "PREPARE FAILED.";
    }
    $stmt->bind_param("s", $_POST['username']);
    $stmt->bind_param("s", $_POST['email']);
    if(!empty($_POST['password'])) {
        $stmt->bind_param("s", $_POST['password']);
    }
    if($session->isSuperuser()) {
        $stmt->bind_param("s", $_POST['usertype']);
    }
    $stmt->bind_param("i", $_POST['userid']);
    if(!$stmt->execute()){
        $err[] = "Execute failed. ERROR: " . $stmt->error;
    }

}

Answer 1

The error you are getting is becauses of these lines:

$stmt->bind_param("s", $_POST['username']);
$stmt->bind_param("s", $_POST['email']);

You can only call bind_param() once and you need to provide the exact number of values as you have placeholders in the SQL. This function is not well designed, which is one of the main reasons people prefer PDO.

To solve the problem you need to dynamically prepare 3 things: placeholders, types and variables to bind. Here is how you could dynamically build such query:

if(empty($err)) {
    $pSETQuery  = '';
    $types = 'sss'; // for the three constant placeholders
    $data = [$_POST['username'], $_POST['email']];
    if(!empty($_POST['password'])) {
        $pSETQuery .= ", password = ?";
        $types .= 's'; //concat one more
        $data[] = $_POST['password'];
    }
    if($session->isSuperuser()) {
        $pSETQuery .= ", usertype = ?";
        $types .= 's'; //concat one more
        $data[] = $_POST['usertype'];
    }
    $data[] = $_POST['userid']; // for UserId
    
    $stmt = $database->prepare("UPDATE user SET username = ?, email = ? $pSETQuery WHERE UserId = ?");
    $stmt->bind_param($types, ...$data);
    $stmt->execute();
}