没有人能告诉我这是什么意思吗

INSERT INTO users
             (first_name,last_name,email,username,password,privilege) 
             VALUES ('s',1,1,1,1,1);

但当我尝试

INSERT INTO users
             (first_name,last_name,email,username,password,privilege) 
             VALUES (1,1,1,1,1,1);

它工作了。我不能插入字符串？什么问题？我的SQLite表看起来像

CREATE TABLE users (
      id          integer PRIMARY KEY AUTOINCREMENT NOT NULL,
      first_name  varchar(50),
      last_name   varchar(50),
      email       varchar(50),
      username    varchar(50),
      password    varchar(50),
      privilege   varchar(50)
    );

我的PHP函数看起来像

public function insert_user($first_name, $last_name
                      , $email, $username
                      , $password, $privilege) 
{
try {
    $db = new SQLite3($this->db_path);

    $db->query(
"INSERT INTO users(first_name,last_name,email,username,password,privilege) 
    VALUES($first_name,$last_name,$email,$username,$password,$privilege);");

     $last_id = $db->lastInsertRowID();
     $db->close();
     return $last_id;
   } catch (Exception $e) {
      echo 'Exception caught: ', $e->getMessage(), "\n";
   }
}