Trying to create PHP CRUD app using PDO but getting Undefined type 'App\PDO' in config file

Entire project - https://github.com/steve-davey/phpsqliteconnect (config file is out of date though)

This is the config file:

<?php

namespace App;

class Config {
   /**
    * path to the sqlite file
    */
    const PATH_TO_SQLITE_FILE = 'db/DeviceAssetRegister.db';

}

/* Database credentials. Assuming you are running MySQL
server with default setting (user 'root' with no password) */
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root');
define('DB_PASSWORD', '');
define('DB_NAME', 'DeviceAssetRegister');
 
/* Attempt to connect to MySQL database */
try{
    $pdo = new PDO("mysql:host=" . DB_SERVER . ";dbname=" . DB_NAME, DB_USERNAME, DB_PASSWORD);
    // Set the PDO error mode to exception
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e){
    die("ERROR: Could not connect. " . $e->getMessage());
}
?>

I don't understand why I get an error for PDO whereas I don't for the index.php file in the directory above?

$pdo = new PDO('sqlite:./db/DeviceAssetRegister.db');

That works absolutely fine! I even get a little pop-up description in VSC that links to the PHP documentation. So why does it throw an error in the other file please?? TIA!