Use Redis scripting to copy an hash to a new key.

redis_hash_copy.php

<?php

require 'autoload.php';

$redis = new Predis\Client('tcp://127.0.0.1', 'dev');

// *** NOTE *** Lua's unpack() has a limit on the size of the table imposed by
// the number of Lua stack slots that a C function can use. This value is defined
// by LUAI_MAXCSTACK in luaconf.h and for Redis is set to 8000, which translates
// to an hash of 4000 elements since HGETALL returns a list that interleaves field
// names and their values.
// See: https://github.com/antirez/redis/blob/b903145/deps/lua/src/luaconf.h#L439

$script = <<<LUA
    local hash = redis.call('hgetall', KEYS[1]);
    if #hash == 0 then
        return { err = 'The key "'..KEYS[1]..'" does not exist' }
    end
    return redis.call('hmset', KEYS[2], unpack(hash));
LUA;


$client->hmset('original', 'foo', 'bar', 'lol', 'wut', 'hoge', 'piyo');
$client->eval($script, 2, 'original', 'copied');
$copied = $client->hgetall('copied');

var_export($copied);

// array (
//   'foo' => 'bar',
//   'lol' => 'wut',
//   'hoge' => 'piyo',
// )