Installing pecl and pear on OS X 10.11 El Capitan, macOS 10.12 Sierra, macOS 10.13 High Sierra (< 10.13.3)

There’s a much easier way — no need to disable SIP or download your own copy:

sudo php /usr/lib/php/install-pear-nozlib.phar -d /usr/local/lib/php -b /usr/local/bin