Why use dollar sign for a variable name?

Answer 1

You use dollar signs in PHP not JS.

Answer 2

Php --> $ is required to start a variable name

JS --> $ is valid character for variable name, even at first position or alone ( as is named the main variable/function of JQuery function )

Answer 3

This is totally related to language design.

$ sign is required to be first character of the variable name in php language and this makes php interpreter faster to lexical analyze your code and determine which word is a variable name.

$ sign isn't required to be first character of the variable name in Javascript but you can use it if you like.