`MultiIndex` class: simple class for keeping multiple indexes on an array of items with easy derived sorting. Originally written to answer [this stackoverflow question](http://stackoverflow.com/q/15800052/1002469).

MultiIndex.php

    class MultiIndex {
    
        protected $array;
        protected $indexes = array();
        protected $indexdefs = array();
        
        function __construct($array, $indexdefs)
        {
            $this->array = $array;
            $this->indexdefs = $indexdefs;
            foreach ($indexdefs as $name => $column) {
                $this->indexes[$name] = $this->makeIndex($column);
            }
        }
        
        function makeIndex($column)
        {
            $index = array();
            foreach ($this->array as $k => $v) {
                $index[$v[$column]] = $k;
            }
            return $index;
        }
        
        function get($key, $index=null)
        {
            $datapk = ($index===null) ? $key : $this->indexes[$index][$key];
            return $this->array[$datapk];
        }
        
        function getIndex($index)
        {
            return $this->indexes[$index];
        }
        
        function getData()
        {
            return $this->array;
        }
        function indexedBy($index)
        {
            $indexed = array();
            $indexedcolumn = $this->indexdef[$index];
            foreach ($this->indexes[$index] as $indexk => $arrayk) {
                $newarray = $this->array[$arrayk];
                unset($newarray[$indexedcolumn]);
                $indexed[$indexk] = $newarray;
            }
            return $indexed;
        }
        
        function sortedBy(/*multisort args*/)
        /* with strings converted to arrays corresponding to index of same name */
        {
            $args = func_get_args();
            foreach ($args as $n => $arg) {
                if (is_string($arg)) {
                    $args[$n] = array_keys($this->indexes[$arg]);
                }
            }
            $sorted = $this->array;
            $args[] = $sorted;
            call_user_func_array('array_multisort', $args);
            return $sorted;
        }
    }

/*
Examples:
  
$data = array(
    array('display_order'=> 0, 'product_id'=>'samgal3',   'display_name' => 'Samsung Galaxy 3'),
    array('display_order'=> 1, 'product_id'=>'motorazrh', 'display_name' => 'Motorola Razr HD'),
    array('display_order'=> 2, 'product_id'=>'a', 'display_name' => 'a'),
    array('display_order'=> 3, 'product_id'=>'c', 'display_name' => 'c'),
    array('display_order'=> 4, 'product_id'=>'d', 'display_name' => 'd'),
    array('display_order'=> 5, 'product_id'=>'b', 'display_name' => 'b'),
    array('display_order'=> 6, 'product_id'=>'q', 'display_name' => 'q'),
    array('display_order'=> 7, 'product_id'=>'f', 'display_name' => 'f'),
);



$dataidx = new MultiIndex($data, array('id'=>'product_id', 'disp'=>'display_order'));
var_export($dataidx->sortedBy('disp', SORT_STRING, SORT_ASC));
var_export($dataidx->indexedBy('id'));
var_export($dataidx->get('samgal3', 'id'));

*/