difference between call by value and call by reference in php and also $$ means?

Question

(1) I want to know what is the difference between call by value and call by reference in php. PHP works on call by value or call by reference?

(2) And a

Answer 1

Call by value: Passing the variable value directly and it will not affect any global variable.

Call by reference: Passing the address of a variable and it will affect the variable.