If you're using brew you need to do nothing. weprovide/valet-plus#127
If you're using the native macos PHP, make sure php is running fine, and see what modules are installed
$php -m
You will see a bunch of modules but they're missing intl
Go here and choose the install script you need to go https://php-osx.liip.ch/
$curl -s http://php-osx.liip.ch/install.sh | bash -s 7.1
Now the default macOS PHP does not have a php.ini. No problem, copy the php.ini.default > php.ini and use that and add the line
extension= /usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so
$php -i | grep intl
This doesn't work on macOS Catalina and PHP 7.3.11. It's giving an error which references the directory in your
extension=line:PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so' (tried: /usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so (dlopen(/usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so, 0x0009): dlopen(): file not found: /usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so), /usr/lib/php/extensions/no-debug-non-zts-20180731//usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so.so (dlopen(/usr/lib/php/extensions/no-debug-non-zts-20180731//usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so.so, 0x0009): dlopen(): file not found: /usr/lib/php/extensions/no-debug-non-zts-20180731//usr/local/php5-7.1.19-20180625-094435/lib/php/extensions/no-debug-non-zts-20160303/intl.so.so)) in Unknown on line 0