I'm adding this bug report to make sure it doesn't get lost.  I would ask that this be marked critial for 4.3.0 release.  The current thread safe Win32 build is _not_ thread safe because it includes BC Math as a built-in extension, but BC Math is not thread safe.

Here is what I said on the php-dev list:
 
> > I'm seeing a memory overrun under PHP 4.3.0pre2 (debug) running under
> > Windows 2000 ISAPI.  [ . . . ]
> > ---------------------------------------
> > C:\Work\php-source\php-4.3.0pre2\ext\bcmath\libbcmath\src\init.c(72):
> > Freeing 0x01B85050 (1 bytes), script=c:\inetpub\wwwroot\test.php
> > Last leak repeated 2 times
> > C:\Work\php-source\php-4.3.0pre2\ext\bcmath\libbcmath\src\init.c(57):
> > Freeing 0x01B84FF8 (29 bytes), script=c:\inetpub\wwwroot\test.php
> > Last leak repeated 2 times
>
>Based on seeing these leaks I disabled BCMath and recompiled PHP.
>Without BCMath active I do not have any of the memory overruns that I
>reported in my previous message.
>
>Looking in the CVS logs for php4/ext/bcmath/bcmath.c I believe there
>may be an issue with the changes introduced in version 1.37 (by Andi,
>who is CCed also).  This patch moved the allocation and freeing of the
>static BC numbers _zero_, _one_, and _two_ to a per-request basis
>instead of at module initilzation and shutdown.  It looks like the
>storage locations for those values are global externs, however, which
>multiple threads are now allocating and deallocating at the same time.
>
>Is there somewhere that I'm not understanding in the code that would
>keep multiple threads from smashing each other here?

And these were Andi's responses:

> > You are right. I screwed up. I have to make these TSRM globals.
> > I'll try and do it tomorrow.
>
> Adding a BCMATH_G() TSRM macro around all instances of _one_, _two_ and 
> _zero_ seems to be quite a pain because it means that libbcmath needs to 
> understand TSRM now.
> On the other hand these three variables need to be per-request because 
> emalloc()'ed memory can't survive in between requests.
> Does anyone have any interesting solutions? If not I guess we can start 
> hacking at the libbcmath sources.