Moving Files/ Temporary Names

I was hoping someone might clarify what may seem like a basic point. I looked in the archives but didnt seem to get my answer.

I am creating a basic set of functions that will form the back bone of an image gallery and a directory handler.

I am trying to crate the function that will allow the administrator to Move a selected file to a different folder.
I have looked at move_uploaded_file () and it seems that the source file to move needs to actually be the file's temp name.
If the file has recently been uploaded then the temp name is accessible via $_Files[] array, but if the file was uploaded say a
week before I assume that the temp name is no longer there.

If I am incorrect, how do I get the temp name for a file uploaded a while ago?

Or is it simpler to copy() the file to its new location and delete the original?

Many Thanks