For arbitrary precision mathematics PHP offers the Binary Calculator which supports numbers of any size and precision, represented as strings.
Since PHP 4.0.4, libbcmath is bundled with PHP. You don't need any external libraries for this extension.
In PHP 4, these functions are only available if PHP was configured with --enable-bcmath. In PHP 3, these functions are only available if PHP was NOT configured with --disable-bcmath.
The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.
The behaviour of these functions is affected by settings in php.ini.
Here's a short explanation of the configuration directives.