Question: Because php 5.6 is EOL from 01.01.2019, it was removed from homebrew.

How can I install it with homebrew even if it is not in core?

Answer: 
I just used **shivammathur/homebrew-php** without any problems.

```
brew tap shivammathur/php
brew install shivammathur/php/php@5.6

```

The repository is on Github:
<https://github.com/shivammathur/homebrew-php>

(the **exolnet/homebrew-deprecated** mentioned in other answers doesn't seem to work anymore)
I tried **exolnet/homebrew-deprecated** but it have some problem with **xdebug** & **pecl**. So I recommend you use official one.

Ref this because of long post:

* <https://devhoi.com/threads/install-brew-deprecated-php-version-php-7-1-5-6-7-0-on-brew-macos.25/>
* <https://devhoi.com/threads/how-to-install-old-deprecated-packages-on-brew.32/>