Won't Query on the UPDATE part of the program - php

Question

I'm having a hard time identifying the cause of the problem of my code, which is, it won't query on the "UPDATE" part but the "SELECT" part does work. when i tried using the print_r function, it gives an errors/warnings namely:
"Warning: mysqli_query(): Couldn't fetch mysqli"** and **"Warning:
mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, null
given"
if(!isset($_POST['n_pass'])&&!isset($_POST['n_pass'])){
if(!isset($_POST['password'])||$_POST['password']==""){
echo 'enter current password';
die;
} else {
include 'include/database.php';
$fname = mysqli_real_escape_string($conn, $_POST['fname']);
$lname = mysqli_real_escape_string($conn,$_POST['lname']);
$email = mysqli_real_escape_string($conn,$_POST['email']);
$username = mysqli_real_escape_string($conn,$_POST['uname']);
$password = mysqli_real_escape_string($conn,$_POST['password']);
//Check if the password is equal to the password inside database
$sql = "SELECT password FROM users where id = $id";
$result = mysqli_query($conn, $sql);
$row = mysqli_fetch_assoc($result);
$cpass = $row['password'];
$verify_pass = password_verify($password,$cpass); //check if current password is equal to the existing password
if($verify_pass != 1){
echo 'incorrect password';
die;
} else {
**//Update Data
$sql="UPDATE users SET firstname=$fname, lastname=$lname, email=$email, username=$username where id=$id";
$result = mysqli_query($conn, $sql);
$row = mysqli_fetch_assoc($result);
print_r($row['firstname']);
die;
header("Location: profile.php?successfullyupdated");
}
}
}

Answer 1

Your query is missing the quotes around the strings:
$sql="UPDATE users SET firstname='$fname', lastname='$lname', email='$email', username='$username' where id=$id";
You can skip only the id field since it is an Integer.
Sidenote: you are wide open to SQL Injections. You should use prepared statements.
There are plenty of resources on google to start with this topic
Finally, note that
$row = mysqli_fetch_assoc($result);
print_r($row['firstname']);
is completely useless since you are not returning anything from the UPDATE query.
You can do:
if(mysqli_query($conn, $sql)){
//query was successful - run your code here for success
}else{
//query failed - run your code here for fail
}