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: 
```
brew tap exolnet/homebrew-deprecated

brew install php@5.6

```

source: <https://joostvanveen.com/a-29/install-and-configure-multiple-php-versions-on-mojave-with-homebrew>
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)