Actually not. I found out that the variable $orientation changes value after the call to setPageSize() function.
I agree with you it makes no sense, but I have printed out this variable before and after the call and found the strange behavior. Thus the $ortmp variable, which is used only for the call to SetPageSize().
Actually, strictly speaking, calling or not the setPageSize with the variable copy in regards to page orientation in my test made no real difference. Even if you leave the original code and add only AddPage('L') (as I did first), it will create the document in landscape mode. I have then tried to use the $orientation variable, so the pdf page orientation would correctly reflect the excel one. I found it was not working, until I found the previously mentioned problem.
I have no idea why this happens, AFAIK a variable passed to a function in PHP should not be changed by the function itself, does it?