First of all the reason it's undeclared, is, I suspect you are posting to the $PHP_SELF thus the first time you get to the page no value is passed and thus the variable is Undeclared (until it actually contains data)
You can control this by adjusting level of error reporting. You can either change that in the PHP.INI or you can suppress the error a couple of ways. You can change
$_GET['Author'] to @$_GET['Author']
or you can use many functions to check the variable such as the isset() or emtpy() function.
These will prevent the throwing of an error (I believe that they are only NOTICE type errors too.)
Hope this helps
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