PHP, Proxies, HTTPS: v2, v3 or v23?!

From a PHP http client, using HTTPS via a proxy, I started getting a "400 bad request" error from Apache. I knew it could work because it worked last week. The apache error log message was:
   Hostname 127.0.0.1 provided via SNI and hostname mytest.local provided via HTTP are different

My first troubleshooting mistake was messing around with server-side settings: I found out what SNI meant, but as far as I could see I wasn't using it. Then, finally, I remembered I could use curl as a test http client, and it was working fine. I removed all my server-side changes and curl was still connecting fine. So now I knew I'd broken something client-side. I added the -v flag to curl to see the exact headers it is sending. We're both sending the same headers.

Finally I remembered I'd changed this line:
  stream_socket_enable_crypto($fp,true,STREAM_CRYPTO_METHOD_SSLv23_CLIENT);

The PHP docs give no guidance on which option to choose, but "v23" sounded like it would work with version 2 or version 3, and maybe do all kinds of auto-negotiation behind the scenes. Which had to be a good thing. I'm sure I'd tested after changing, but I must have tested non-HTTPS or without the proxy by mistake. When I changed back to this line, everything worked again:
  stream_socket_enable_crypto($fp,true,STREAM_CRYPTO_METHOD_SSLv3_CLIENT);

I hope that helps someone, as google was no use for me (all the hits about the SNI name and HTTP name difference were due to Apache being case-sensitive about the name comparison, which was not the problem here).

By the way if you want to know how to do http connections, with a proxy, using PHP, supporting both HTTP and HTTPS, I've described it here.