On macOS, it's common for users to have multiple versions of OpenSSL installed. The PHP_SETUPUP_OPENSSL() Autotools function we run from config.m4 in driver versions before 1.4.0 is prone to picking up headers for the OpenSSL version that ships with macOS, while a version of PHP installed by Homebrew likely links and runs with Homebrew's version of OpenSSL.

To ensure that the correct version of OpenSSL is picked up during compiling, users may need to use the --with-openssl-dir=/path/to/openssl option for configure, as bjori describes in mongodb/mongo-php-driver#188.

As of 1.4.0, which includes PHPC-888, we should also inform users of the --with-mongodb-ssl=LIB option for configure and PKG_CONFIG_PATH environment variable, which controls pkg-config. Lastly, we would do well to suggest that Homebrew users simply use a Homebrew formula to install the driver instead of PECL.