Description:
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While trying to find out what CURLOPT_BINARYTRANSFER did exactly, I found that it isn't used at all. I looked around in the ext/curl directory in trunk for uses of the PHP_CURL_BINARY and PHP_CURL_ASCII constants, and found that they're only used for setting the ch->handlers->write->type value, but this value is never used anywhere.

It seems the last use of the constants has disappeared in r211314 ("fix bug #37061 (curl_exec() doesn't zero-terminate binary strings) - we get the data length from cURL, so it's binary safe.
fix leak appearing when re-using curl handle").

I'm not 100% sure my analysis is correct, but if it is, I guess the (code for) the option should be removed and the documentation updated. there's no need for them to be zero-terminated

It actually makes sense not to distinguish between binary and non-binary results: A normal string is just a special case of binary string, so always returning a "binary string" should work just fine. AFAIK strings always have a length associated with them, so zero bytes could occur within any PHP string (it's just that using strings with zero bytes in them don't always work with external C functions that expect zero terminated strings...).