The current 2.0.1-r1 ebuild of pecl-rrd has the following in the ebuild: USE_PHP="php5-6 php7-0" resp. USE_PHP="php7-0" We should add php7-1 to this list as php-7.1 has been in portage for some time now. I have been running pecl-rrd with USE_PHP="php5-6 php7-0 php7-1" resp. USE_PHP="php7-0 php7-1" for some weeks now and can confirm that it builds and runs just fine with the newer version of php.
pecl-rrd revbumped to include 7.1 support