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

It means $($a), so its the same as $name (Since $a = 'name'). More explanation here What does $$ (dollar dollar or double dollar) mean in PHP?