The base_url function now checks the FORCE_HTTPS_ON environment variable first before evaluating proxy headers and server variables. This ensures that when the environment variable is set, HTTPS is forced regardless of other conditions, providing a reliable way to enforce HTTPS in proxy environments.

Additionally, update test cases to cover all scheme detection scenarios including standard HTTPS, proxy headers (X-Forwarded-Proto, Cloudflare), and the FORCE_HTTPS_ON environment variable.