Getting started:
About the site:
About the optimizations:
About the warnings:
Google purports that copying global variables to local variables uses extra memory.

Because there are many reasons why you might want to copy a global variable, the program simply warns you of any place you do, so you can asses whether it is necessary or not. The following is an example of why you might want to copy a variable:

$foo = $_POST["age"];
echo "You are ".$_POST["age"]." years old and will be ".$foo." next year";
$bar = $_POST["age"];

Yes, you could get the same example as above without the variable foo at all, but if it was much more complex and involved unknown numbers, you wouldn't be able to.