我的代码跳过了if-else块的if语句，仅执行else块。虽然早些时候工作良好

Question

我已经设置了一个register.php页面来在我的网站上注册用户，当他们提交表单时，他们应该被重定向到login.php页面，如果有任何错误，它应该显示消息“出了点问题，请重试后来”。它已经在几天前工作了，但是由于某种原因它总是显示错误消息。

// Define variables and initialize with empty values

$ username = $ password = $ confirm_password = $ firstname = $ lastname = $ emailaddress = $ phonenumber = $ address =“”; $ username_err = $ password_err = $ confirm_password_err = $ firstname_err = $ lastname_err = $ emailaddress_err = $ phonenumber_err = $ address_err =“”;

//提交表单时处理表单数据 if（$ _ SERVER [“ REQUEST_METHOD”] ==“ POST”）{

// Validate username
if(empty(trim($_POST["username"]))){
    $username_err = "Please enter a username.";
} else{
    // Prepare a select statement
    $sql = "SELECT id FROM users WHERE username = ?";

    if($stmt = mysqli_prepare($link, $sql)){
        // Bind variables to the prepared statement as parameters
        mysqli_stmt_bind_param($stmt, "s", $param_username);

        // Set parameters
        $param_username = trim($_POST["username"]);

        // Attempt to execute the prepared statement
        if(mysqli_stmt_execute($stmt)){
            /* store result */
            mysqli_stmt_store_result($stmt);

            if(mysqli_stmt_num_rows($stmt) == 1){
                $username_err = "This username is already taken.";
            } else{
                $username = trim($_POST["username"]);
            }
        } else{
            echo "Oops! Something went wrong. Please try again later.";
        }
    }

    // Close statement
    mysqli_stmt_close($stmt);
}

// Validate password
if(empty(trim($_POST["password"]))){
    $password_err = "Please enter a password.";     
} elseif(strlen(trim($_POST["password"])) < 6){
    $password_err = "Password must have atleast 6 characters.";
} else{
    $password = trim($_POST["password"]);
}

// Validate confirm password
if(empty(trim($_POST["confirm_password"]))){
    $confirm_password_err = "Please confirm password.";     
} else{
    $confirm_password = trim($_POST["confirm_password"]);
    if(empty($password_err) && ($password != $confirm_password)){
        $confirm_password_err = "Password did not match.";
    }
}

// Validate first name
if(empty(trim($_POST["firstname"]))){
    $firstname_err = "Please enter your first name.";     
} else{
    $firstname = trim($_POST["firstname"]);
     }

   if(empty(trim($_POST["lastname"]))){
    $lastname_err = "Please enter your last name.";     
} else{
    $lastname = trim($_POST["lastname"]);
     }

      if(empty(trim($_POST["emailaddress"]))){
    $emailaddress_err = "Please enter your email address.";     
} else{
    $emailaddress = trim($_POST["emailaddress"]);
     }

      if(empty(trim($_POST["phonenumber"]))){
    $phonenumber_err = "Please enter your phone number.";     
} else{
    $phonenumber = trim($_POST["phonenumber"]);
     }

     if(empty(trim($_POST["address"]))){
         $address_err= "Please enter your address.";
     }else{
         $address=trim($_POST["address"]);
     }


// Check input errors before inserting in database
if(empty($username_err) && empty($password_err) && empty($confirm_password_err) && empty($firstname_err) && empty($lastname_err) && empty($emailaddress_err) && empty($phonenumber_err) && empty($address_err))
{

    // Prepare an insert statement
    $sql = "INSERT INTO users (username, password, firstname, lastname, emailaddress, phonenumber, address) VALUES (?, ?, ?, ?, ?, ?,?)";

    if($stmt = mysqli_prepare($link, $sql)){
        // Bind variables to the prepared statement as parameters
        mysqli_stmt_bind_param($stmt, "sssssis", $param_username, $param_password, $param_firstname, $param_lastname, $param_emailaddress, $param_phonenumber, $param_address);

        // Set parameters
        $param_username = $username;
        $param_password = password_hash($password, PASSWORD_DEFAULT); // Creates a password hash
        $param_firstname = $firstname;
        $param_lastname = $lastname;
        $param_emailaddress = $emailaddress;
        $param_phonenumber = $phonenumber;
        $param_address= $address;

        // Attempt to execute the prepared statement
        if(mysqli_stmt_execute($stmt)){
            // Redirect to login page
            header("location: login.php");
        } else{
            echo "Something went wrong. Please try again later.";
        }
    }

    // Close statement
    mysqli_stmt_close($stmt);
}

// Close connection
mysqli_close($link);

} ？>