Error: Could not load database driver type mysqli!

<?php 
class DB { 
	private $driver; 
 
	public function __construct($driver, $hostname, $username, $password, $database) { 
		$file = DIR_DATABASE . $driver . '.php'; 
 
		if (file_exists($file)) { 
			require_once(VQMod::modCheck($file)); 
 
			$class = 'DB' . $driver; 
 
			$this->driver = new $class($hostname, $username, $password, $database); 
		} else { 
			exit('Error: Could not load database driver type ' . $driver . '!'); 
		} 
	} 
 
	public function query($sql) { 
		return $this->driver->query($sql); 
	} 
 
	public function escape($value) { 
		return $this->driver->escape($value); 
	} 
 
	public function countAffected() { 
		return $this->driver->countAffected(); 
	} 
 
	public function getLastId() { 
		return $this->driver->getLastId(); 
	} 
} 
?>