undefined function: bcdiv()

I've just finished migrating my system to dynamic version 4.0.0 php
(apache 1.3.12, redhat 6.1, Kernel 2.2.12-20).

Most everything works correctly (including phplib, happily). But in going
through my various scripts, I encountered the following error:

Fatal error: Call to undefined function: bcdiv() in
/home/httpd/html/album/picadded.php on line 63

the offending line reads as follows:
$newwidth=bcdiv(($width*$ratio),1,0);

Any ideas?

Thanks,
Arlon