"builtin: not found" when I source a script via PHP passthru()
Running a script as . ~/.rvm/scripts/rvm does not use its shebang line. It sources the script into the current shell. The current shell for passthru would seem to be /bin/sh (which is reasonable most things default to using /bin/sh).
If you want the script's shebang to be used then you need to leave off the sourcing ./source command.