To expand on what nicoSWD stated about this function.
Any script working with the temporary file $_FILES[]['tmp_name'] should call this function.
In any case where the script is modified to unlink(), rename() or otherwise modify the file that IS NOT move_uploaded_file() will not have the upload checked.
Likewise, most file operations are cached in PHP, therefore there should be minimal performance hit running is_uploaded_file before move_uploaded_file, since it will usually used a cached result for the latter.
The security benefits outweigh the microsecond difference in performance in any event, and should universally be used as soon as the $_FILES array is first entered into an application. While there may not be an immediate issue, code evolves and could quickly change this fact.
