Make a delegate function that reads variables from the environment and

[ivstefano]

https://github.com/zigivicious/42/blob/master/setup/configs/dev.php

$configurations = [
    ConfigurationVariables::DATABASE => function () {
        $host     = getEnvironmentVar(EnvironmentVariables::DEVELOPMENT_HOST);
        $dbName   = getEnvironmentVar(EnvironmentVariables::DEVELOPMENT_DBNAME);
        $username = getEnvironmentVar(EnvironmentVariables::DEVELOPMENT_USERNAME);
        $password = getEnvironmentVar(EnvironmentVariables::DEVELOPMENT_PASSWORD);
        return new DBConfig($host, $dbName, $username, $password);
    }
]
$injector->delegate('DatabaseConfig', $configuration[ConfigurationVariables::DATABASE]);

Add thanks to Dan Ackerman