To turn on debug mode go to:
Set define('_PS_MODE_DEV_', false); --> define('_PS_MODE_DEV_', true);
To upgrade PHP Memory Limit
vim /etc/php5/apache2/php.ini --- Go to Memory Limit.
To check PHP Config create a file with the following code.
<?php phpinfo(); ?>
Another way to raise you your servers performance is to create a SWAP file, which uses a space on the hard drive to serve as RAM when the regular RAM is full.
A good tutorial on this is here: https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-ubuntu-14-04
Using the command:
sudo fallocate -l 4G /swapfile