In order to use the OpenSSL functions you need to install the » OpenSSL library. PHP 7 requires at least OpenSSL >= 0.9.8. Other versions (PHP >= 7.1.0) require OpenSSL >= 1.0.1. Only PHP >= 8.1 supports OpenSSL >= 3.0.
You are strongly encouraged to use a maintained OpenSSL version, otherwise your web server could be vulnerable to attack.