The manual is a little shy on explaining that output buffers are nested, and that "turns off output buffering" means turning off the highest nested buffer. See ob_get_level (for a useful function, but still no explanation)
// ob_get_clean() returns the contents of the last buffer opened. The first "blah" and the output of var_dump are flushed from the top buffer on exit
Prior to realising this, I had thought PHP's ob functionality left more to be desired. I *really* wish I knew earlier.