move_uploaded_file（）不会移动文件

I have a class by the name of Input. Within this class is a function called iData. This function is used to get file upload data, here's the function.

public static function iData($item, $spec = '')
{
    if (isset($_FILES[$item])) {
        if ($spec == '') {
            return $_FILES[$item];
        }else{
            $f = $_FILES[$item];
            if($f[$spec]) {
                return $f[$spec];
            }
        }
    }
    return false;
}

But in my controller(By the way I'm using the MVC design pattern), when I tell it to move the file it doesn't. Here's my controller's code.

if (!empty(Input::iData('file', 'name'))) {
    if (is_dir('assets/')) {
        if (move_uploaded_file(Input::iData('file', 'name'), 'assets/')) {
            echo "File was moved!";
        } else {
            echo "File failed to move!";
        }
    }
} else {
    echo "Please include a file to upload!";
}

My page echos, "File failed to move!". So if the directory exists and I know that the Input class and iData function return the name, what is wrong?

All is fixed. Much thanks to Hobo Sapiens

I was supposed to use the tmp_name instead of the name. And also, I left the desired file name out of the move_uploaded_file's second parameter.