mysqli_query插入没有工作，mysqli_error（）没有显示任何内容

Question

我尝试使用mysqli_query()进行插入，但它似乎无法正常工作，我无法弄明白。 mysqli_error()也没有帮助我（它返回NULL）。这是我的代码：

<?php
include('db_fns.php');

function register() {

// register new person with db
// return true or error message

$fname = $_POST['first_name'];
$lname = $_POST['last_name'];
$email = $_POST['email'];
$password = $_POST['password'];
//$education = $_POST['education'];
//$faculty = $_POST['faculty'];
//$department = $_POST['department'];

  // connect to db
  include('db_fns.php');

  // check if email is unique
  $result = mysqli_query($db, "select * from users where email='".$email."'");
  if (!$result) {
    throw new Exception('Could not execute query');
  }

  if (mysqli_num_rows($result)>0) {
    throw new Exception('Το email χρησιμοποιείται. Παρακαλώ διαλέξτε κάποιο άλλο.');
  }

  // if ok, put in db

  $result = mysqli_query($db,"insert into users values
                         ('".$fname."', '".$lname."', '".$email."', sha1('".$password."'))");
                         mysqli_query($db,"COMMIT");
  if ( false===$result ) {
  printf("error: %s\n", mysqli_error($db));
}

  return true;
}
register();
?>

这是db_fns.php：

<?php
$db = mysqli_connect ("localhost","root","","sfhmmy10") or die ("Could not connet to MySQL");
mysqli_select_db ($db,"sfhmmy10") or die ("No database");
mysqli_set_charset($db, "utf8");
?>