Functions from this package are useful for number theory applications.
For example, in two-keys cryptography.

See /tests/RSA.php in the package for example of simple implementation of
RSA-like cryptoalgorithm.  See http://chat.finalcombat.com/vayala/big_int/ page
for more complex implementation of RSA-like crypto, which supports key
generating, encrypting/decrypting, signing and validating of sign.

The package has many bitset functions, which allow to work with arbitrary
length bitsets.

This package is much faster than bundled into PHP BCMath and consists almost
all functions, which are implemented in PHP GMP extension, but it needn't any
external libraries.

WWW: http://pecl.php.net/package/big_int/