Increase memory_limit on Plesk VPS

Connect via Shell using Putty.

Change memory_limit for an individual site

Note: replace mydomain.com with your actual domain.

cd /var/www/vhosts/mydomain.com/conf/

using an editor (vi, nano, pico), create vhost.conf with the following content:

<Directory /var/www/vhosts/mydomain.com/httpdocs>
php_value memory_limit 32M
</Directory>

Save the file, then run >

/usr/local/psa/admin/sbin/websrvmng --reconfigure-vhost --vhost-name=mydomain.com

Change memory_limit globally for all site(s)

The default value for memory_limit is configured in /etc/php.ini.

cd /etc

using an editor (vi, nano, pico) edit php.ini, save the file, then restart Apache:

service httpd restart

Now your memory for all site(s) should be 32M or whatever value you've configured.