PHP's array_map, but instead of mapping values, maps keys.

array_key_map.php

<?php
/**
 * Array map, but maps values to new keys instead of new values
 * @return array same arrays with keys mapped
 */
function array_map_key($callback,$array)
{
    $out=array_reduce($array, function ($carry,$val) use ($array,$callback){
        $key=call_user_func($callback,$val);
        $carry[$key]=$val;
        return $carry;
    });
    return $out;
}