I've seen that PHP has an extensive function list for sorting arrays and either I've missed it 10 time over or they just don't have the one I want.

I have an array that looks like this:
$arr[book01][title]
$arr[book01][author]
$arr[book01][isbn]
$arr[book01][releasedate]
$arr[book01][...]

$arr[book02][title]
$arr[book02][author]
$arr[book02][isbn]
$arr[book02][releasedate]
$arr[book02][...]

and I would like to be able to sort my array by the author. I've tested a few of the functions on the link provided above but most of what I get is it either sorts by

book01, book02, ...

or

book01, title
book02, title

Is there anything I can do here? (thanks in advance)