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

I know this is probably old thread but just in case this also works well:

id -Gn "username"|grep -c "groupname"

if any number > 0 is returned then user is a member of that group.