PHP Warning: mysqli_real_escape_string() expects parameter 1 to be mysqli, null given

Question

<?php

session_start();
$username = "";
$email = "";
$errors = [];

$conn = new mysqli('localhost', 'rcbmjkoa_ITN', 'kmWEC2018', 'registration');
// REGISTER USER
if (isset($_POST['reg_user'])) {
// receive all input values from the form
$username = mysqli_real_escape_string($db, $_POST['username']);
$email = mysqli_real_escape_string($db, $_POST['email']);
$password_1 = mysqli_real_escape_string($db, $_POST['password_1']);
$password_2 = mysqli_real_escape_string($db, $_POST['password_2']);

// form validation: ensure that the form is correctly filled ...
// by adding (array_push()) corresponding error unto $errors array
if (empty($username)) { array_push($errors, "Username is required"); }
if (empty($email)) { array_push($errors, "Email is required"); }
if (empty($password_1)) { array_push($errors, "Password is required"); }
if ($password_1 != $password_2) {
array_push($errors, "The two passwords do not match");
}

// first check the database to make sure
// a user does not already exist with the same username and/or email
$user_check_query = "SELECT * FROM users WHERE username='$username' OR email='$email' LIMIT 1";
$result = mysqli_query($db, $user_check_query);
$user = mysqli_fetch_assoc($result);

if ($user) { // if user exists
if ($user['username'] === $username) {
array_push($errors, "Username already exists");
}

if ($user['email'] === $email) {
array_push($errors, "email already exists");
}
}

// Finally, register user if there are no errors in the form
if (count($errors) == 0) {
$password = md5($password_1);//encrypt the password before saving in the database

$query = "INSERT INTO users (username, email, password)
VALUES('$username', '$email', '$password')";
mysqli_query($db, $query);
$_SESSION['username'] = $username;
$_SESSION['success'] = "You are now logged in";
header('location: index.php');
}
}

// ...

// LOGIN USER if not working re

Answer 1

Have you figured it out? well, just in case you haven't; the mysqli_real_escape function expects the first argument passed to be a mysqli connection resource, your connection resource was $conn, but you passed $db, which is an undefined identifier. Replace all occurrence of $db with $conn, or change the connection variable to $db, this way less lines to replace : )

The same case also for your calls to mysqli_query function for checking if user/e-mail already exists, and for inserting new user data, you also passed $db as connection resource there.