$array = array(3, 5, 2, 8);

$array = array(
    array('foo' => 'bar', 'baz' => 42),
    array('foo' => ...,   'baz' => ...),
    ...
);

function cmp(array $a, array $b) {
    if ($a['foo'] < $b['foo']) {
        return -1;
    } else if ($a['foo'] > $b['foo']) {
        return 1;
    } else {
        return 0;
    }
}

usort($array, 'cmp');