Unable to install PHP 7 with Homebrew on macOS Sierra

Right I've figured it out. I'm posting the steps to successfully install the require components below in case anybody else stumbles upon this issue:

1). make sure Homebrew is up to date:

brew update

2). Fix the PHP 7 error by making sure Xcode is installed and then typing the following into the Terminal:

brew install apr apr-util
sudo mkdir -p /Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.12.xctoolchain/usr/local/bin/
sudo ln -s /usr/local/opt/apr/bin/apr-1-config /Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.12.xctoolchain/usr/local/bin/
sudo ln -s /usr/local/opt/apr-util/bin/apu-1-config /Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.12.xctoolchain/usr/local/bin/

3). Run the following command (otherwise the PHP installation will complain about missing libz):

xcode-select --install

4). You can now install PHP 7 with Homebrew:

brew install homebrew/php/php70

It may also be important for those using the macOS-delivered installation of Apache with PHP via Homebrew that (quote from Homebrew php70 install):

With the release of macOS Sierra the Apache module is now not built by default. If you want to build it on your system you have to install php with the --with-apache option. See brew options php70 for more details.