php inside [collapse] tags?

my page is blowing up when i try to wrap a php statement inside [collapse] tags. the page is set for php and works normally when the php is outside the [] tags. but as soon as i try to put the php statement inside the tag, the thing won't save properly and goes into an infinite loop of some kind. i can escape out of it and revert to previous version of the page to recover, so no worries there.

but can you tell me if this behavior is by design? should php work inside collapse tags?

i guess maybe it is a problem cause the php wants to run and print results when the page is loaded, but can't do so properly because [collapse] is preventing that part of the page from loading?

the php statement is this (it calls and displays an ad via OpenX module): <?php echo openx_invoke('ad-name');?>

thank you.