Media Temple has rather nice article on the topic of modifying php.ini file and enabling php5 on a per-domain basis but they don’t address this particular issue.

If you need to use a separate php.ini file for a particular domain in Plesk, the easy way is to copy the php.ini file into the current domain’s bin folder, and then restart the server using the below commands. The php executable looks in it’s own directory first for a valid php.ini file before looking in the other locations, and before the default php.ini file is loaded.

In our case, we needed to do this because we needed to specify a unique session.save_path for each domain, but there are certainly countless other reasons why you would want to use a special php.ini file per domain.

Create local version of php.ini for this domain
cp /opt/php51/etc/php5/fastcgi/php.ini /var/www/vhosts/example.com/bin/php.ini

Restart Plesk web server
/usr/local/psa/admin/sbin/websrvmng -a -v