Make code more memory efficient, or shorter //rename files    
    if(!empty($_FILES[$desktop_fieldname_1280x800]['name'])){
                file_exists($desktop_filename_1280x800 = mb_strtolower($desktop_dir_1280x800 . 'wall_desktop_1280x800_' .
                $_FILES[$desktop_fieldname_1280x800]['name']));
            }   
    if(!empty($_FILES[$desktop_fieldname_1366x768]['name'])){
                file_exists($desktop_filename_1366x768 = mb_strtolower($desktop_dir_1366x768 . 'wall_desktop_1366x768_' .
                $_FILES[$desktop_fieldname_1366x768]['name']));
            }   
    if(!empty($_FILES[$desktop_fieldname_1920x1080]['name'])){  
                file_exists($desktop_filename_1920x1080 = mb_strtolower($desktop_dir_1920x1080 . 'wall_desktop_1920x1080_' .
                $_FILES[$desktop_fieldname_1920x1080]['name']));    
            }


With the above code, basically what I'm doing is renaming the files properly + consistently. What my code seems to be bit redundant to me, I think this can be done with some kind of loop or arrays, but I'm a newbie and not sure how it will be. So would like to ask your help to make this code more memory efficient, or shorten the code in some way.
The code does not rename any files, it modifies the values of certain variables.

Using assignments as a parameter to your functions is confusing.

You call file_exists but do nothing with it.  It is supposed to be used to check for the existence of a file. Something like:

if (!file_exists($destination))
{
    rename($source, $destination);
}


To answer more you would need to show how the desktop_dir_xxx variables are calculated.