Многомерный массив вида несколькими ключами

Это можно сделать с помощью usort . Аргумент $ cmp_function может иметь следующий вид:

function my_sorter($a, $b) {
    $c = strcmp($a['state'], $b['state']);
    if($c != 0) {
        return $c;
    }

    $c = strcmp($a['event_type'], $b['event_type']);
    if($c != 0) {
        return $c;
    }

    return strcmp($a['date_start'], $b['date_start']);
}

Для произвольного количества полей в PHP 5.3 вы можете использовать замыкания для создания функции сравнения:

function make_cmp($fields, $fieldcmp='strcmp') {
    return function ($a, $b) use (&$fields) {
        foreach ($fields as $field) {
            $diff = $fieldcmp($a[$field], $b[$field]);
            if($diff != 0) {
                return $diff;
            }
        }
        return 0;
    }
}

usort($arr, make_cmp(array('state', 'event_type', 'date_start')))

Для произвольного количества полей разных типов в PHP 5.3:

function make_cmp($fields, $dfltcmp='strcmp') {
    # assign array in case $fields has no elements
    $fieldcmps = array();
    # assign a comparison function to fields that aren't given one
    foreach ($fields as $field => $cmp) {
        if (is_int($field) && ! is_callable($cmp)) {
            $field = $cmp;
            $cmp = $dfltcmp;
        }
        $fieldcmps[$field] = $cmp;
    }
    return function ($a, $b) use (&$fieldcmps) {
        foreach ($fieldcmps as $field => $cmp) {
            $diff = call_user_func($cmp, $a[$field], $b[$field]);
            if($diff != 0) {
                return $diff;
            }
        }
        return 0;
    }
}

function numcmp($a, $b) {
    return $a - $b;
}
function datecmp($a, $b) {
    return strtotime($a) - strtotime($b);
}
/**
 * Higher priority come first; a priority of 2 comes before 1.
 */
function make_evt_prio_cmp($priorities, $default_priority) {
    return function($a, $b) use (&$priorities) {
        if (isset($priorities[$a])) {
            $prio_a = $priorities[$a];
        } else {
            $prio_a = $default_priority;
        }
        if (isset($priorities[$b])) {
            $prio_b = $priorities[$b];
        } else {
            $prio_b = $default_priority;
        }
        return $prio_b - $prio_a;
    };
}

$event_priority_cmp = make_evt_prio_cmp(
    array('meeting' => 5, 'party' => 10, 'concert' => 7), 
    0);

usort($arr, make_cmp(array('state', 'event' => $event_priority_cmp, 'date_start' => 'datecmp', 'id' => 'numcmp')))

class Sort {
    private $actual_order = 'asc';
    private $actual_field = null;

    public function compare_arrays($array1, $array2) {

        if ($array1[$this->actual_field] == $array2[$this->actual_field]) {
            return 0;
        }
        elseif ($array1[$this->actual_field] > $array2[$this->actual_field]) {
            return ($this->actual_order == 'asc' ? 1 : -1);
        }
        else {
            return ($this->actual_order == 'asc' ? -1 : 1);
        }

    }


    public function order_array(&$array) {

        usort($array, array($this, 'compare_arrays'));

    }


    public function __construct ($field, $actual_order = 'asc') {
        $this->actual_field = $field;
        $this->actual_order = $actual_order;
    }

}

// use

$sort = new Sort ("state");

$sort->order_array($array);