@airsine Thanks for posting the issue. We don’t seem to have many Mint users and didn’t notice that obviously they have moved to default to PHP 7.2 being pre installed. At least that is my interpretation of the error without actually having looked into it.
If you are not bound to use Mint then see George’s and Wayne’s answers.
If you want to stick to it, try running the installer as php_ver=7.2 ./installfog.sh (note this is just from the top of my head, no promise this is working)