Don't copy variables for no reason.
Sometimes PHP novices attempt to make their code "cleaner" by copying predefined variables to variables with shorter names. What this actually results in is doubled memory consumption, and therefore, slow scripts. In the following example, imagine if a malicious user had inserted 512KB worth of characters into a textarea field. This would result in 1MB of memory being used!
$description = $_POST['description'];
Now I would never question the almighty Google, but the Rasmus Lerdorf taught me that PHP uses copy-on-write. Quoting from his O'Reilly Programming PHP book:
When you copy a value from one variable to another, PHP doesn't get more memory for a copy of the value. Instead, it updates the symbol table to say "both of these variables are names for the same chunk of memory."
So who's right? I tend to believe M. Lerdorf since he pretty much invented PHP but like I said before I'm not an expert and my book is pretty old so (PHP 4.1.0) so maybe that has changed since (although I doubt it)...
[We] use bad software and bad machines for the wrong things. -- R.W. Hamming