So, I need to research this to figure out what the heck is going on in this little script and how it works, but in the mean time - just for me and you - here is what works:
cd '/home/sitename/public_html' ; php -q 'cron.php' > /dev/null 2>&1
but this also works (I have this on another site):
php /home/sitename/public_html/cron.php