Download the SDK for your system from the SDK page.

Unpack the archive containing the SDK:

shell> tar xzf php-ext-couchbase-$system-$arch.tar.gz

The resulting directory includes a file couchbase.so which is the PHP extension for the SDK. Note the path to this file.

Find the php.ini file for your PHP interpreter. Try:

shell> phpi -i | grep ini

The output provided will have configuration file that is loaded for PHP, for instance:

Loaded Configuration File => /private/etc/php.ini

Open the php.ini file and add the path to your couchbase.so file. For example:

extension=/path/to/couchbase.so