A customer of mine uses an old and depricated version of php. My system on the other hand is up to date and uses php7. Nearly all the code is deprecated and the project did not compile. So I had to install the outdated php5.6 somehow. This tutorial shows you how to install php5.6 on arch.
Luckily we are not the only ones. There is an AUR package for php56.
Check the installation.
Edit the server conf:
Test the configuration.
Add a php info file on the local test server:
Restart / start the apache server:
Check if the "correct" php version is running.
The next step would be to convince the customer to upgrade his php...