I personally prefer uppercase because it's more consistent with other
constants. What source says lowercase is faster?

Steve Rude wrote:
  

So I was trying to research a coding standard that I don't see
documented anywhere.
What is the standard about NULL, TRUE and FALSE.  According to the coder
module it says to use lowercase because it is faster, but throughout
core there is uppercase.


e.g. in the system.module (not to mention the trailing comma is missing...)

/**
* Implementation of hook_menu().
*/
function system_menu($may_cache) {
 $items = array();

 if ($may_cache) {
   $items[] = array('path' => 'system/files', 'title' => t('File
download'),
     'callback' => 'file_download',
     'access' => TRUE,
     'type' => MENU_CALLBACK);

I know this is a very minor point, but we are trying to match our
internal coding standards with that of Drupal.

Thanks!

Steve Rude