if i have

define("_DEFINED_CONSTANT", 0)

function xyz ($committype = _DEFINED_CONSTANT)
{
  if ($committype = _DEFINED_CONSTANT)
  {
    ...do stuff...
  }
}

then the if statement fails to recognise that when a 0 = 0, the statement should be true, instead it returns zero, which is rather anoying...

Its a pretty obvious problem - you can try it yourself just put 

$pug = 0
if ($pug = 0)
{ echo "foo"; }

an it won't recognise it...
(it crashes if you ask it for 'if (0 = 0) {...}' for some reason - perhaps another bug?

IcK