致命错误：在非对象错误上调用成员函数 query()

Question

I have a small problem. I'm trying to make a simple register/login system with sessions and I got this error:

Fatal error: Call to a member function query() on a non-object in C:\\xampp\\htdocs\\members\\includes\\login.inc.php on line 9

This is the relevant line of code:

$result = $conn->query($sql);

The first time I tried it was working.

The rest of the code:

<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
    $email      = $_POST['email'];
    $password   = md5($_POST['password']);
    $sql = "SELECT email, password FROM member WHERE email = '$email' AND password = '$password'";
    $result = $conn->query($sql);

I also have db.php, which is used to connect the MYSQL and everything inside it is fine.

Answer 1

I cannot understand why, the first time I tried it was working I guess and now this kind of error.

I'm also having the db.php which is used to connect the MYSQL and everything inside is fine.. could someone explain me why I keep facing this error ?

I'm going to speculate. I'm speculating that you have a separate file (probably called db.php ) which "handles" the setting up of the database connection. I'm further going to speculate that you've a chain of files which are require() (or include() )'d into your web app.

I've seen this more times than I care to recall. It's a very old fashioned way of separating code into logical chunks inside PHP - which needs to be left in the past.

I'm speculating that you were previously defining $conn in another script which was included (or required) before this code. A global variable, which had was dependency later in the code execution. Invisible to the file it was declared in.

That's the problem . The quick/hack fix is to rename $conn or the restore the original declaration of it and make sure it's global and make sure it is included before this code is ran.

The proper fix (IMHO) is to look at using a framework (Laravel, Lumen, CodeIgniter, Yii, there are many - take your pick) and read up on the topics of dependency injection, autoloading and namespacing. Think about why global variable declarations make for unmaintainable code.

If you're really reluctant to go with a full framework, at the very least have a look at some database-abstraction libraries like doctrine (and it's sub-library dbal ) which can easily be auto-loaded into your project via composer .

Answer 2

As Sascha already pointed out, $conn might be either not defined at all, or it's not an object (hence the error message).

From the code sample you have provided, it's actually a bit hard to tell what kind of connection object you might be using, but I think it's save to say that in your case it might be either PDO or mysqli .

For the sake of simplicity, let's stick with mysqli. A working code sample based on mysqli would look like this (shortened example taken from the docs cited above):

$conn   = new mysqli("localhost", "my_user", "my_password", "world");
$result = $conn->query($sql);

Though you really should go for so-called prepared statements, as your code right now is prone to SQL injection as wally already stated.

I would have linked wally's answer and provide you with a link to the PHP docs relating to prepared statements, but apparently, my lack of reputation points don't allow me to, so just do a quick Google search for PHP & prepared statements.

Answer 3

The database connection file has to be added at the beginning of the file.

The present format is easy.

<?php  $mysqli = new mysqli("localhost", "Userid", "password", "database name"); if ($mysqli->connect_errno) {
    echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error; }
?>

Answer 4

At the beginning：

require 'db.php'; $conn = new db();