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

Let's define a function:

function f($a) {
  $a++;
  echo "inside function: " . $a;
}

Now let's try calling it by value(normally we do this):

$x = 1;
f($x);
echo "outside function: " . $x;

//inside function: 2
//outside function: 1

Now let's re-define the function to pass variable by reference:

function f(&$a) {
  $a++;
  echo "inside function: " . $a;
}

and calling it again.

$x = 1;
f($x);
echo "outside function: " . $x;

//inside function: 2
//outside function: 2

You can pass a variable by reference to a function so the function can modify the variable. More info here.