mysqli_stmt :: bind_param（）：变量数与准备好的语句错误中的参数数不匹配

Question

if(isset($_POST['submit'])){
    $password = md5($_POST['password']);
    $username = $conn->escape_string($_POST['user']);
    $middlename = $conn->escape_string($_POST['middlename']);
    $surname = $conn->escape_string($_POST['surname']);
    $lastname = $conn->escape_string($_POST['lastname']);
    $stmt = $conn->stmt_init();

    /*foreach($_POST as $post){
        if($post == '')
            $error['login'] = "No field should be left blank";
    }*/

    if(strlen($_POST['user']) < 5){
        $error['username'] = 'username is too short';
    } else{
        $stmt->prepare('SELECT username from admin where `username` = ?');
        $stmt->bind_param('s', $_POST['user']);
        $stmt->bind_result($user);
        $stmt->execute();
        $stmt->fetch();
        if($_POST['user'] == $user){
            $error['username'] = 'This username has been registered already';
        }
        $stmt->free_result();
    }

    if($_POST['password'] != $_POST['confirmPassword']){
        $error['password'] = 'password does not match';

    }elseif(strlen($_POST['password']) < 6){
        $error['password'] = 'password is too short';
    }

    if(strlen($_POST['surname']) < 2){
        $error['surname'] = 'surname is too short';
    }

    if(strlen($_POST['middlename']) < 2){
        $error['middlename'] = 'surname is too short';
    }

    if(strlen($_POST['lastname']) < 2){
        $error['lastname'] = 'surname is too short';
    }

    if(count($error) == 0){
        include('./../core/connect.php');
        $sql = "INSERT INTO admin (username, password, surname, middlename, lastname) VALUES (?, ?, ?, ?, ?)";
        $stmt->prepare($sql);
        $stmt->bind_param("sssss", $username, $password, $surname, $firstname, $lastname);
        $stmt->execute();
        $success = $stmt->affected_rows;
    }}

我在使用代码时遇到问题，并且我搜索了google和文档，但均未成功。我确保在准备好的语句中没有使用带问号（？）的单引号，并且尝试将var_dump $ _POST超级数组使用，但仍然找不到错误。我唯一注意到的是我的$ _POST ['user']写为 'user'=>＆string'stanley'（length = 6）