Description:
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Problem with chgrp: it doesn't change group for a secondary group.

Documentation on http://php.net/chgrp says "other users may change the group of a file to any group of which that user is a member." So, this implies it can change it not only to the primary group but also to any secondary group identfied by /etc/group. Also, the function would be pretty much useless then as only the super user could use it.

That doesn't work. I added the user www-data to the group vmail and then tried to chgrp a file that is owned by www-data (which is also the pool user) to vmail. I get a warning: chgrp(): Operation not permitted

Might be an fpm problem? e.g. fpm not being aware that user also belongs to other groups as group = www.data ?

I did not check if chgrp works with primary group. That was not the goal.

I tested on the shell by su-ing to www-data and was able to change the group this way. So, the user www-data does have enough permissions. php-fpm just doesn't seem to recognize that.

Expected result:
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chgrp function should be able to chgrp to *any* (=every) group the owner of the fpm process belongs to.