PHP 7.1, I did instruction from official website:
wget http://packages.couchbase.com/releases/couchbase-release/couchbase-release-1.0-6-amd64.deb
sudo dpkg -i couchbase-release-1.0-6-amd64.deb
Will install or upgrade packages
sudo apt-get update
sudo apt-get install libcouchbase-dev build-essential php-dev zlib1g-dev
sudo pecl install couchbase