Do we need both server scripting like (PHP)and client scripting languages like (JAVASCRIPT) in building a website?..Is it a ""MUST LEARN SITUATION""....

I mean does both the langages related to each other ..or we can just learn one language and take that knowledge to build a website.Putting HTML and CSS aside.
Please can someone clearify my confusion in plain and in a simple way.Thank You..:)
I ll be waiting for the replies...