Apparently, passing the php.ini file in argument of the php built-in webserver command line is the solution, like this:
php -S localhost:8000 -t /srv/http/TradeinnDev/killred -c /etc/php/php.ini
From the man page of PHP:
--php-ini path|file
-c path|file Look for php.ini file in the directory path or use the specified file