Working Config in Plesk 9.x
This config worked for me in Plesk 9.x. You may need to create a vhost.conf if doesn't exist. Hope this helps.
/srv/www/vhosts/domain.com/conf # cat vhost.conf
<Directory /srv/www/vhosts/domain.com/httpdocs>
<IfModule sapi_apache2.c>
php_admin_flag engine...