چرا در تابع اپدیت زیر با ارور مواجه می شوم

[ramezani72]

 public function update(){

  global $database;





  $sql="UPDATE users SET";

  $sql.="username= '" . $database->escape_string($this->username) ."',";

  $sql.="password= '" . $database->escape_string($this->password) ."',";

  $sql.="first_name= '" . $database->escape_string($this->first_name) ."',";

  $sql.="last_name= '" . $database->escape_string($this->last_name) ."'";

  $sql.="where id=" . $database->escape_string($this->last_name);





  $database->queryi($sql);





  return (mysqli_affcted_rows($database->conection)==1) ? true : false;

















 }

have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '= 'example_username',password= 'sdfsdf',first_name= 'reza',last_name= 'williams'' at line 1