Check if a user is in a group

Question

I have a server running where I use php to run a bash script to verify certain information of a user. For example, I have a webhosting server set up, and in order to be able to

Answer 1

Sounds like an another answer:

username='myuser'
if grep -q -E "^customers:.*[:,]$username(,.*|\b)" /etc/group; then
    echo 'true'
else
    echo 'false'
fi

As reported by sputnick the output of the groups command may depend on your OS. I am not sure how this code is going to perform, but most probably it will do better.