Although the extension conditionally includes libcrypto headers at build time if the OpenSSL extension is available, it does not use PHP_SETUP_OPENSSL() to link to it. This could lead to undefined symbol errors if PHP itself does not happen to load OpenSSL at runtime.
Pecl extension doesn't load when using PHP 5.6.31. I have confirmed the mongo.so exists and is included in the loaded php.ini file. I see no mention of any issues in the logs. We have no issues if we spin up another box using php version 5.6.28