You'll need to provision multiple tools to get this all working. Here's what I can determine right now, just from what you've posted so far:
- httpd.conf(5). Don't provision the loopback network interface as an "external" interface. You'll confuse us ... and possibly confuse yourself. The loopback network interface is an internal loop on your network stack. Hence its name. (Yes, OpenBSD can redirect traffic in and out of this pseudo network interface with pf(4), but let's not get tangled in those weeds. Please.)
- PHP. There are multiple versions of PHP in the ports/packages system. Each version requires installation and provisioning of matching-version "extensions" for select extended services. One of those extensions is php-mysqli, the MySQL / MariaDB database extension for PHP. It's missing, and needs to be both installed via pkg_add(1), and then provisioned via ln(1), as described in /usr/local/share/doc/pkg-readmes/php-<your installed release of php>.