mysqli_fetch_array($result, MYSQLI_NUM); - php

Question

This mysqli_fetch_array not returning values. Checked it for SQL errors, none found. some error in PHP or functions I guess. Please help me correct it.
$login = login($username, $password);
if ($login == false) {
$errors[] = "That username and password combination is incorrect";
}
// validate login
function login($username, $password){
$user_id = id_from_username($username);
echo $user_id;
$password = md5($password);
$username = sanitize($username);
$query = "SELECT `password` FROM `user_data` WHERE `username` = '$username' ";
$result = mysqli_query($conn,$query);
$row = mysqli_fetch_array($result, MYSQLI_NUM);
if ($row[0] == $password) {
echo "Login successful";
return $user_id;
}
else{
echo "Login not successful";
return false;
}
}
// user id from username
function id_from_username($username){
$username = sanitize($username);
$query = "SELECT `user_id` FROM `user_data` WHERE `username` = '$username'";
$result = mysqli_query($conn,$query);
$row = mysqli_fetch_array($result, MYSQLI_NUM);
return $row[0];
}

Answer 1

EDIT: As Lawrence correctly pointed out I missed the variable scope issue. mysqli_query does not have access to $conn variable.
Try to check the number of returned rows in the $result:
echo $result->num_rows;
Or print it to the log if you have some. Not sure how you are checking mysqli_fetch_array does not return values - try var_dump() or print_r().
Few further recommendations:
Both queries in your question select from the same table - user_data - it is pretty inefficient to do it separately (unless id_from_username is commonly used elsewhere), I'd merge that into one query to select it at once:
SELECT user_id, password FROM user_data WHERE ...
Using mysqli_query and concatenating user input into the query is usually a bad idea. Not sure what your sanitize function does but I'd still use variable binding instead, even mysqli supports that with mysqli_prepare, see here.
Based on your code you are storing passwords in database using md5. This is a very bad practice. PHP provides very nice functions password_hash and password_verify since PHP 5.5.0, they will handle password hashing for you and make your application a lot more secure.