PHP 5.6 (incorrectly) redefined the meaning of STREAM_CRYPTO_METHOD_SSLv23_CLIENT.
Using SSLv23 now explicitly removes TLS support, leaving only SSLv2 and SSLV3 available.
MongoDB has never supported SSLv2, but did support SSLv3 until 2.4.13/2.6.6 which explicitly disabled SSLv3.
This leaves no available protocols in PHP 5.6.0-5.6.6.
This has been reported as https://bugs.php.net/bug.php?id=69195