What do you already know and what's your goal? Find employment? (then in which country?)
For doing a project on your own?
PHP is more mature and more spread (most popular language), rails will get you faster to your goal :)
I use both, if you are looking for a dev house job go with rails as loads of dev houses are looking for rails at the moment in my Area, Bristol&Bath(England)
Spend a week on each, figure out which one you enjoy more and dive deeper into it. Neither are so obviously advantageous that the answer is clear cut.