error with prepare() function - php

Question

I have this login page that I writes it while watching a tutorial on Udemy. His code works properly, but in my code (the same), I have the following error:
Fatal error: Call to a member function prepare() on a non-object
This is the code:
<?php
//$var = 'This is our first web app page';
//echo $var;
//Connection Variables:
$dbhost = "localhost";
$dbname = "graphic_db";
$dbuser = "root";
$dbpass = "root";
//Connection to SQL:
$conn = new PDO("mysql:host=$dbhost; dbname=$dbname", $dbuser, $dbpass);
//Error messagin enabled:
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
//Adding a character set:
$conn = exec("SET CHARACTER SET utf8mb4");
$user = '';
$pass = '';
$sum = 0;
$error_msg = "Please type a username and a password";
if(isset($_POST['login_submit']))
{
//Start a session:
session_start();
$user = $_POST['username'];
$pass = $_POST['password'];
if(empty($user) && empty($pass))
{
echo $error_msg;
$pass = '';
}
if(empty($user) || empty($pass))
{
echo $error_msg;
$user = '';
$pass = '';
}
if(!empty($user) && !empty($pass))
{
//SQL:
$query = $conn->prepare("SELECT * FROM login WHERE user = :u AND password= :p LIMIT 1");
$query->bindParam(":u", $user);
$query->bindParam(":p", $pass);
//Execute query:
$query->execute();
$number_rows = $query->fetch(PDO::FETCH_NUM);
if($number_rows>0)
{
echo $user;
$_SESSION = $user;
$_SESSION = $pass;
header("Location: /pages/home.php");
}
else
{
echo "Invalid username or password";
header("Location: index.php");
}
//echo $user;
}
}
if(!isset($_POST['login_submit']))
{
echo "Login button not clicked";
}
?>

Answer 1

You destroy the $conn object with this statement:
$conn = exec("SET CHARACTER SET utf8mb4");
Replace it by:
$conn->exec("SET CHARACTER SET utf8mb4");
Note that from PHP 5.3.6 onwards, you can set the character set in the connection string, like this:
$conn = new PDO("mysql:host=$dbhost; dbname=$dbname; charset=utf8", $dbuser, $dbpass);
The separate exec call is then no longer necessary.