what is the difference between client-side programming language and server-side programming language?
Why javascript is only a client-side programming language, but PHP is a server-side programming language?
I am a newbie starting web development and need some advice.

Javascript can be used for either client or server-side usage, but it is most commonly used for client side stuff, especially graphics-intensive stuff that should most appropriately be run on the client. Other than that, it is a general-purpose web programming language. PHP really is designed for server-side programming, and can send html or javascript code the the client for processing and display there. It (PHP) is commonly misused and that leads to insecure web applications. Properly used, it is a very good OOP (object-oriented programming) language with a lot of C++ flavor. I actually used it at Nokia to write a web-based cell-phone emulator!

FWIW, these days, it is better for more processing to be run on the client rather than the server? Why you may ask? Because servers have to server possibly thousands or millions of users over a short period, so reducing the processing load there allows a better interactive experience, especially given the powerful processors and adequate memory most current user-facing systems possess.