*
* @param int $number Number of filenames to write
* @param string $format Optional, must be understood by ImageMagick, such as 'jpg' or 'gif'
- * @param string $dir Directory, optional (will default to current working directory)
+ * @param string|null $dir Directory, optional (will default to current working directory)
* @return array Filenames we just wrote
*/
function writeImages( $number, $format = 'jpg', $dir = null ) {