I use Box to build the Phars.
To install the Box binary, use the instructions they supply:
curl -LSs https://box-project.github.io/box2/installer.php | php
From the root of the project, you can then do:
./box.phar build -c builds/box-0.1.1.json
The Box project itself appears to have been abandoned, and good-quality forks seem to exist. However the old Box project is well starred on GitHub, and works fine for me.