php array sorting with accents

Use multi-byte string functions. There is a function called strcoll which seems to suit your needs.

More info:

• On how to sort an array of UTF-8 strings
• How to sort an array of UTF-8 strings?

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EDIT: added Peter's working code, below

setlocale(LC_COLLATE, 'sk_SK.utf8');

usort($fb_friends['data'], 'custom_sort');

function custom_sort($a, $b) {
    return strcoll ($a['last_name'], $b['last_name']);
}

foreach ($fb_friends['data'] as $friend) {
    echo '<br>';
    echo $friend['name'];
}