# This might be a longer process but it worked for me
# firstly completely remove the current PHP present in your OS (ubuntu)
# command to remove > sudo apt-get purge php7.*
# next command > sudo apt autoremove -y
# now install the php version you want with the command > sudo apt-get install php7.2 libapache2-mod-php7.2 php7.2-mcrypt php7.2-mysql