PHP v Javascript

I'm still developing my first PHP script (TellAFriend) and have realised that everything I'm doing via PHP can be done with Javascript (data validation, showing errors to the user, email generation, etc). However, I want some advice on what people think is the best solution! If I ensure that the client has JavaScript enabled then there should be no problems using it? For a busy professional website is it conventional wisdom to perform as much processing on the client side (i.e. JavaScript) to remove as much burden from the server? Therefore if something can be done via JavaScript rather than PHP then should I use JavaScript? Thanks James