The userland workaround is rather trivial though, isn't it?

list(, $value1, $value2) = array_values(unpack("CnCnC", $packet));

array_values() is quick, but I guess your performance worry is the needless 
creation of the associative array in the first place?

I was under the impression that it's not safe to depend on the order of values in an associative array, i.e., that array_values(array("a" => "b", "c" => "d")) is free to return array("d", "b") if it likes. Is that untrue in the current engine?

And yeah, performance is some question. Especially since I am using unpack for network packets, and the faster the better, though I haven't done any kind of benchmark to see if this is a bottleneck at all.

That has never been true in any PHP version. Right from day one PHP arrays have 
been ordered and you have always been able to rely on that order.

I stand corrected!

That having been said, it's still a bit annoying to have to write the array_values() every time, and it does feel a bit wasteful to create an associative array, then create array from just its values, just to extract it into what's essentially another hash table (the symbol table) again. A comment in ext/standard/pack.c suggests the original author of the function rejected the ordered-array approach because it was error-prone, which now seems a little odd to me given your explanation.