How do you sort a multidimensional array by primary and secondary key? For example, assuming the following array:
$result = array();
$result[0][\"prio\"] =
array_multisort() should give you the functionality that you need... use Example #3 from the PHP documentation as the basis for your sort, although you'll need to convert those date strings to a datestamp value before executing the sort.
If this data has come from a database query though, it's far easier to sort it within the sql query used to retrieve the information.