variable practices

[presence]

I understand that it's common practice to assign the values of superglobals to new variables when working with them. Ie:

$user_id = $_SESSION['user_id'];

What are the advantages of doing this, and what are the costs?

[NogDog]

The advantage is that you now have a variable that's easier to type and read.

The disadvantages are that you have extra commands to execute as well as more variable names and values to store in RAM. Most of the time this will add only a tiny bit of overhead to your script, but might be noticeable if any of those variables are holding huge amounts of data. Also, the new variables will not have global scope, unless you also add a global command to make them global. (Then again, I suppose the global/non-global aspect might be a feature in certain circumstances that I've yet to run into.)