Per How do I uncompress a tarball that uses .xz?, I tried
tar xf php-5.6.0RC4.tar.xz and
tar -xJf php-5.6.0RC4.tar.xz and in both cases I get the following:
tar (child): xz: Cannot exec: No such file or directory tar (child): Error is not recoverable: exiting now tar: Child returned status 2 tar: Error is not recoverable: exiting now
It's this php-5.6.0RC4.tar.xz.
I'm running Ubuntu 14.04 LTS.