Sorts $database on multiple columns.
This function returns array.
$myDatabase = ptb_sort($myDatabase, 'name ASC, birth DESC');
You can't sort on link (@fieldname) nor multi (%fieldname) field (but you can on linked field).
If no last sorting sequence (i.e. ASC or DESC) is given, ASC is used:
$myDatabase = ptb_sort($myDatabase, 'name ASC, birth');
is equal to:
$myDatabase = ptb_sort($myDatabase, 'name ASC, birth ASC');
If you select any records from your database, you can do sorting in one go with ptb_select. In fact, ptb_select simply uses ptb_sort internally.