Vercel PHP serverless functions failing to connect to Supabase Postgres

Problem

I am building a web application using a modern frontend stack (Vite, TailwindCSS, Flowbite, and SweetAlert2) powered by a PHP backend. Initially, this project was developed and tested successfully on my local machine using a Laragon/MySQL setup.

To deploy the application on Vercel, I decided to upgrade my database and migrate from MySQL to Supabase (PostgreSQL). While the application and the Supabase connection work perfectly on my local environment, the database connection completely fails after deploying to Vercel, despite the frontend loading successfully.

Current Behavior

Frontend loads perfectly, but API calls to /api/beneficiaries.php return a 500 status.
My DB diagnostic endpoint confirms the following:
- db_connection=pgsql
- use_supabase=true
- host/port/db/user/password are all set correctly
- db_sslmode=require
- pdo_pgsql_loaded=true

Despite this, it still fails with the error:

Error: Could not connect to database. Please check your configuration.

Expected Behavior

PHP API functions deployed on Vercel should successfully connect to the Supabase Postgres database and return data, exactly as they do on my local machine.

Steps to Reproduce

Deploy project to Vercel with PHP functions (vercel-php@0.7.4) and Vite-built frontend.
Set production environment variables:
- DB_CONNECTION=pgsql
- DB_HOST
- DB_PORT=5432
- DB_DATABASE
- DB_USERNAME
- DB_PASSWORD
- DB_SSLMODE=require
- APP_ENV=production
- ENV_KEY=production
Open the diagnostic endpoint: https://dole-system.vercel.app/api/auth.php?db_test=1

The output returned is:

{
  "success": false,
  "error": "Could not connect to database..."
}

Project Information

URL: https://dole-system.vercel.app
Frontend: Vite, TailwindCSS, Flowbite, SweetAlert2
Backend: PHP API on Vercel Serverless (vercel-php@0.7.4)
Database: Supabase Postgres (Direct connection)
Runtime config confirmed via script:
- db_connection=pgsql
- use_supabase=true
- db_sslmode=require
- pdo_pgsql_loaded=true

Main Question

What are the most likely causes when PHP on Vercel reports all DB variables as present and the pgsql extension as loaded, but the PDO connection still fails to reach Supabase Postgres?

Is there any Vercel-specific networking, SSL, or connection-string requirement for connecting to Supabase using PHP serverless functions?