Call to a member function prepare() on a non-object with PDO
require_once("connect.php");

function get_username($uid){
  $stmt = $pdo->prepare("SELECT username FROM users WHERE uid= ?");

try {
    $stmt->execute(array($uid));
} catch (PDOException $e) {
    echo $e -> getMessage(); exit;
}

      $row = $stmt->fetch();
      return($row['username']);
}

$id = 1;
echo get_username($id);

Call to a member function prepare() on a non-object

$dsn = 'mysql:dbname=test_db;host=127.0.0.1';
$user = 'test_user';
$password = 'test_pass';

try {
    $pdo = new PDO($dsn, $user, $password);

} catch (PDOException $e) {
    echo 'Connection failed: ' . $e->getMessage();
    exit;
}

$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

error_reporting(E_ALL);
session_start();

function get_username($uid){
  global $pdo;
  $stmt = $pdo->prepare("SELECT username FROM users WHERE uid= ?");

function get_username($uid, $pdo){

...
...
}

get_username( $id, $pdo);