Floating Point numbers don't work on my setup.
Example:

$mystring="1.25";
$mydouble=(double) $mystring;
print($mydouble);

prints out "1", instead of "1.25". 

I did several tests, even with code from documentation, and I always get integer numbers, both on my box and my ISP box (similar config). On other boxes it seems to work nicely.
It worked nicely even on my box with PHP3 installed.

I also tried to add BCMATH support, but it still doesn't work.
I configured my php4 with:
./configure --with-apxs=/usr/sbin/apxs --with-mysql --with-pdflib --disable-debug