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 behaviour of these functions is affected by settings in php.ini.
For further details and definition of the PHP_INI_* constants see ini_set().
Here is a short explanation of the configuration directives.
Number of decimal digits for all bcmath functions.