If you installed OJS and the screen comes up blank or with error 500, just go into the folder called "cache" and delete it all, and make sure you chmod 777 it. IE: cd /var/www/ojs/cache rm -rf * chmod 777 .
How to force a specific version of php on linux: Let's say you want to force the linux machine to use php 7.2 specifically Do this: sudo add-apt-repository -y ppa:ondrej/php sudo apt-get install php7.2 php7.2-cli php7.2-common now tell apache: sudo a2dismod php5.6 sudo a2enmod php7.2 sudo service apache2 restart now tell bash: sudo update-alternatives --set php /usr/bin/php7.2 sudo update-alternatives --set phar /usr/bin/phar7.2 sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.2