PHP将字符串（姓）作为数组插入数据库

Question

我正在向个人PHP项目添加注册功能，但是在将数据插入MySQL数据库时，它会将姓氏记录为数组而不是字符串。数据库中的列显示单词“ Array”而不是真实名称，并且echo mysqli_info($conn);返回“ Array in string conversion in...。”是否有解决方法？还是我做错了什么？

数据库结构 Database Structure

表格 enter image description here

数据库结果 enter image description here

HTML

<form action="includes/signup-handler.php" method="post" class="signup-form">
  <div class="form-group">
    <input type="text" name="firstName" id="firstName" class="form-control" placeholder="first name">
  </div>
  <div class="form-group">
    <input type="text" name="lastName" id="lastName" class="form-control" placeholder="last name">
  </div>

  <div class="form-group">
    <input type="email" name="email" id="email" class="form-control" placeholder="email address">
  </div>

  <div class="form-group">
    <input type="password" name="password" id="password" class="form-control" placeholder="password">
  </div>

  <div class="form-group">
    <input type="password" name="repeatPassword" id="password" class="form-control" placeholder="confirm password">
  </div>

  <button type="submit" class="btn btn-success" name="submit">Sign up</button>

</form>

还有PHP处理程序；

if(isset($_POST['submit'])){
    require 'connection.php';

    $firstName = $_POST['firstName'];
    $lastName = $_POST['lastName'];
    $email = $_POST['email'];

    //Check for validation
    } elseif(!filter_var($email, FILTER_VALIDATE_EMAIL) && filter_var($email, FILTER_SANITIZE_EMAIL)){
        header("Location: ../register.php?error=invalidemail");
        exit();
    }
    elseif (!preg_match("/^[a-zA-Z0-9]*$/", $firstName, $lastName)) {
        header("Location: ../register.php?error=invalidname&email=".$email);
    exit();
    }

    //Create prepared statements for validation
    else {
        $selectQuery = "SELECT * FROM users WHERE email=?";
        $statement = mysqli_stmt_init($conn);
        if(!mysqli_stmt_prepare($statement, $selectQuery)){
            header("Location: ../register.php?error=sqlerror");
            exit();
        }else{
            mysqli_stmt_bind_param($statement, "s", $email);
            mysqli_stmt_execute($statement);
            mysqli_stmt_store_result($statement);
            $resultCheck = mysqli_stmt_num_rows($statement);
            if ($resultCheck > 0) {
                header("Location: ../register.php?error=usertaken&firstName=".$firstName."&lastName=".$lastName."&email=".$email);
                exit();
            }else{
                $insertQuery = "INSERT INTO users (firstName, lastName, email) VALUES (?, ?, ?);";
                $statement = mysqli_stmt_init($conn);
                    if(!mysqli_stmt_prepare($statement, $insertQuery)){
                        header("Location: ../register.php?error=sqlerror");
                        exit();
                    }else {
                        mysqli_stmt_bind_param($statement, 'sss', $firstName, $lastName , $email);
                        mysqli_stmt_execute($statement);
                        header("Location: ../register.php?error=success");
                        //echo mysqli_info($conn);
                        // var_dump($lastName);
                        exit();
                    }
            }
        }
    }
    mysqli_stmt_close($statement);
    mysqli_close($conn);
}else{
    header("Location: ../register.php");
    exit();
}