In order to use the OpenSSL functions you need to install the » OpenSSL library. PHP 5 requires at least OpenSSL >= 0.9.6. However later PHP 5 versions have some compilation issues and should be used at least with OpenSSL >= 0.9.8 which is also a minimal version for PHP 7.0. Other versions (PHP >= 7.1.0) require OpenSSL >= 1.0.1.
You are strongly encouraged to use the most recent OpenSSL version, otherwise your web server could be vulnerable to attack.