Q:

SQLSTATE[HY000] [1049] Unknown database ' register_login'

I am not sure what is going on here.  I have changed the database name.  I have gone into phpMyAdmin and copy and pasted the name just to make sure the database was right.  here is my code. 
class DB {
  private static $_instance = null;
  private $_pdo,
          $_query,
          $_error = false,
          $_results,
          $_count = 0;

  private function __construct() {
    try{
      $this->_pdo = new PDO('mysql:host='.  Config::get('mysql/host') . '; dbname= ' .  Config::get('mysql/db'), Config::get('mysql/username'), Config::get('mysql/password'));
    } catch(PDOException $e){
      die($e->getMessage());
    }
  }

  public static function getInstance(){
    if(!isset(self::$_instance)){
      self::$_instance = new DB();
    }
    return self::$_instance;
  }
}

Here is the class for config,
class Config {
  public static function get($path = null){
    if($path){
      $config = $GLOBALS['config'];
      $path = explode('/', $path);

      foreach($path as $bit){
        if(isset($config[$bit])){
         $config = $config[$bit];
          }
        }
      return $config;
    }
    return false;
  }
}

Here is the init.php file I have made. 
session_start();

$GLOBALS['config'] = array(
  'mysql' => array(
    'host' =>  '127.0.0.1',
    'username' => 'root',
    'password' => 'root',
    'db' => 'register_login'
  ),
  'remember' => array(
    'cookie_name' => 'hash',
    'cookie_expiry' => 604800
  ),
  'session' => array(
    'session_name' => 'user'
  )
);

spl_autoload_register(function($class) {
  require_once 'classes/' .  $class . ' .php';
});

require_once 'functions/sanitize.php';

And here is the index
require_once 'core/init.php';

$user = DB::getInstance()->get('users', array('username', '=', 'Taylor'));
 echo $user;
if(!$user->count()){
  echo 'No user in table';
} else {
  foreach($user->results() as $user){
    echo $user->username, '<br>';
  }
}

A:

SQLSTATE[HY000] [1049] Unknown database ' register_login' in your error. 
You see that space before register_login?  It's looking for a database of that name BUT with a space before it, so of course it doesn't exist, in turn throwing you that error. 
There shouldn't be any spaces in here dbname= ' .  Config::get('mysql/db') so you need to change that to dbname='. Config::get('mysql/db') and make  sure there are no other spaces on that line. 

The PDO DSN should not contain any spaces.