// Working as of 2023

As homebrew removed the extra php repository containing a version with xdebug already installed, you have to install it manually.

Summary

1. brew install <php version> for php
2. update your path
3. pecl install xdebug for xdebug

Full step-by-step example

# update homebrew
brew update

# install a version of php, e.g. 7.0
brew install php@7.0

# now they tell you how to link it, in my case
echo 'export PATH="/usr/local/opt/php@7.0/bin:$PATH"' >> ~/.bash_profile
echo 'export PATH="/usr/local/opt/php@7.0/sbin:$PATH"' >> ~/.bash_profile

# reload the file with the updated path, so we can use pecl
source ~/.bash_profile

# check that the path is to the correct php executable,
# and pecl is available
which pecl
# returns: /usr/local/opt/php@7.0/bin/pecl

# install xdebug, see https://xdebug.org/docs/install#pecl
pecl install xdebug

# check that everything worked
php --version
# should show a xdebug version
# like:  with Xdebug v2.6.0, Copyright (c) 2002-2018, by Derick Rethans

The pecl install xdebug step above ended with

Build process completed successfully
Installing '/usr/local/Cellar/php@7.0/7.0.30/pecl/20151012/xdebug.so'
install ok: channel://pecl.php.net/xdebug-2.6.0
Extension xdebug enabled in php.ini

So I didn’t even need to enable the xdebug.so in php.ini.

If you need a special version of xdebug (e.g. your IDE doesn’t like the 3.x.x versions), you can install a specific version xdebug-$VERSION, e.g. pecl install xdebug-2.9.8.
You can find them on the list of available versions (Thanks Bower)