Você pode usar putenv () , adicionando "/ foo":
php > putenv('PATH=' . getenv('PATH')); print_r(getenv('PATH'));
/home/jpic/env/bin:/home/jpic/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:/usr/local/bin:/usr/local/sbin
php > putenv('PATH=' . getenv('PATH') . ':' . '/foo'); print_r(getenv('PATH'));
/home/jpic/env/bin:/home/jpic/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/foo