I just managed to get php to build on my musl system:
Next up should be to try to get the phoronix test suite running.
An issue that I still have is chainloading from my syslinux on my Arch root partition (/dev/sda1) to my "sabotage linux" partition (/dev/sda4). Somehow it does not get it and the syslinux info is not that informative unfortunately. The system does however run nicely in a chroot so some preliminary tests of musl libc with pts could probably be done already but ideally I should run a clean system without overhead from a host system.