A shiv for dotenv files from existing _ss_environment.php files in SilverStripe 4

mysite-env-shiv.php

<?php
/**
 * Imports SS_* constants into the environment so dotenv can read them.
 *
 * You should include this from your mysite/_config.php file BEFORE ConfigureFromEnv.php
 *
 * This file is temporary - until SilverStripe Platform supports auto-generation
 * of .env files instead of/as well as _ss_environment.php
 */
if (!file_exists(BASE_PATH . '/.env')) {
    // Define paths to look for _ss_environment in
    $paths = [
        BASE_PATH . '/_ss_environment.php',       // In the project root
        BASE_PATH . '/../_ss_environment.php',    // SSP already deployed
        BASE_PATH . '/../../_ss_environment.php'  // SSP during deployments
    ];

    // Find the first environment file that exists
    foreach ($paths as $path) {
        if (file_exists($path) && is_readable($path)) {
            require_once $path;

            // Get the user defined constants that start with SS_
            foreach (get_defined_constants(true)['user'] as $constantName => $constantValue) {
                if (substr($constantName, 0, 3) !== 'SS_') {
                    continue;
                }
                // Import into the environment so the first execution will work. You can also
                // write this to a .env file, if required
                putenv(sprintf('%s=%s', $constantName, $constantValue));
            }

            break;
        }
    }
}
unset($path, $paths, $constantName, $constantValue);