The composer.phar
file is supposed to work out of the box when getting executable flags for the user it is trying to run, i.e. /path/to/composer.phar
will work without adding PHP. On Windows, .phar files probably should get associated with the installed PHP version when being opened.
But one important thing: Composer does nothing related to ensuring uninterrupted production service! That's not it's task. Always think about what happens if anything that Composer is doing fails. There are so many components that might become unavailable, starting with packagist.org, Github/Bitbucket/younameit, accidentially grabbing broken released versions during update which will disable the update process entirely, and so on...
Think about the risks you add when trying to add such attention-free cron-based update step.
Another thing is that you have to ensure that the tools needed to get the software onto the machine are installed. This means you must have at least ZIP tools available, tar/gz as well, and depending on whether you install branches (you should not do this, even more so with automated unattended updates) you will need Git, Mercurial and SVN installed on the production machine (and this might also be true if you only install tagged versions). So there isn't just the dependency to PHP, but to at least these five tools, too.