Buildkit and php binary is called php56
I just took a look and the reference to php in the bash scripts seems to be hardcoded.  I'm assuming there's already a php app with a different version, or else you could just put a symlink from php56 to php somewhere in your path.  So I think your best bet is to:


Create that symlink anyway, but don't put it in your default path.  E.g. ln -s /usr/bin/php56 /my/folder/php.
Add a line to civibuild (or call from a wrapper script) that puts the symlink at the BEGINNING of your path, so it's found before your regular php.  E.g. PATH="/my/folder:$PATH".


By putting the symlink in the path only when civibuild is called it won't cause interference with your other applications that rely on a different PHP version to run.