'Class X not found' error

Code:

<?php namespace App\Libraries;

class BNCdb
{
    private $db = NULL;

    public function __construct()
    {
        try {
            $db = new PDO('informix:host=dev-bnc3-db; database=development; server=devbnc, bncdev, bncdev!');
         $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
     } catch (PDOException $e) {
            log_message('error', $e->getMessage());
        }
    }
}

When I instantiate this class I get a 'Class App\Libraries\PDO not found' error. Why?

PDO is enabled. I'm using PHP 7. Why is it looking in App\Libraries?

Simpler is always better