Check if specific input file is empty

You can check by using the size field on the $_FILES array like so:

if ($_FILES['cover_image']['error'] == 4 || ($_FILES['cover_image']['size'] == 0 && $_FILES['cover_image']['error'] == 0))
{
    // cover_image is empty (and not an error), or no file was uploaded
}

(I also check error here because it may be 0 if something went wrong (ie. a file was selected, but there’s no data received). I wouldn’t use name for this check since that can be overridden). error with a value of 4 is UPLOAD_ERR_NO_FILE, so we can check for that too.