<?php
// A function to sort by last name.
function lastNameSort($a, $b) {
    $aLast = end(explode(' ', $a));
    $bLast = end(explode(' ', $b));

    return strcasecmp($aLast, $bLast);
}

// The array of data.
$array = array(
    27 => 'Sarah Green',
    29 => 'Adam Brown',
    68 => 'Fred Able'
);

uasort($array, 'lastNameSort');

print_r($array);
?>

Array
(
    [68] => Fred Able
    [29] => Adam Brown
    [27] => Sarah Green
)