a single equals sign in PHP is an assignment operator, it says that the container on the left should be filled with the value on the right.
so when you say
you are saying $id gets the value contained in the id field of the $_GET superglobal. following it up with
says "nevermind, go ahead and replace what we just stored in $id with what's in $currentid (which is nothing).
assignment is always right to left. Just like when you're doing algebra and you're isolating a variable; you may start with
10=4x + 2
in proper form it ends up being
x = 2
because you're assigning the expression on the right to the variable on the left