How to sort date array in php?

Because array items is string, you need to convert them to date and then comparing to sort. The usort() sort array using custom function that is a good sort function for this case.

$arr = array('11-01-2012', '01-01-2014', '01-01-2015', '09-02-2013', '01-01-2013');    
function date_sort($a, $b) {
    return strtotime($a) - strtotime($b);
}
usort($arr, "date_sort");
print_r($arr);

Check result in demo

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?

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'];
}

An quick fix, using the previous numerically ordered array, could have been:

// Save the keys
$keys = array_shift($data);

/* here, do the sorting ... */

// Then apply the keys to your ordered array
$data = array_map(function ($item) {
    global $keys;
    return array_combine($keys, $item);
}, $data);

But let's update my previous function:

function mult_usort_assoc(&$arr, $max_index = false, $index = false) {

    function mult_usort_callback($a, $b, $max_index, $index) {
        $max_index = $max_index ?: key(end($a));    // If not provided, takes last key
        $index     = $index ?: array_keys($a)[0];   // If not provided, takes first key

        // When data are equal, sort by next key only if not after the $max_index
        if ($a[$index] == $b[$index]) {
            $next = array_keys($a)[array_search($index,array_keys($a))+1];
            if ($index !== $max_index && !is_null($next)) {
                return mult_usort_callback($a, $b, $max_index, $next);
            } else {
                return 0;
            }
        }
        return $a[$index] > $b[$index] ? 1 : -1;
    }

    /* This part have been improved as we shouldn't use create_function()
       This function is now usable in PHP 7.2 */
    usort($arr, function ($a, $b) use ($max_index, $index) {
        return mult_usort_callback($a, $b, $max_index, $index);
    });
}

Usage and output:

mult_usort_assoc($data, 'Site');
echo '<pre>' . print_r($data, true) . '</pre>';

Used functions in this answer:

• array_combine()
• array_keys()
• array_search()
• is_null()