These functions work using » mcrypt. To use it, download libmcrypt-x.x.tar.gz from » http://mcrypt.sourceforge.net/ and follow the included installation instructions.
As of PHP 5.0.0 you will need libmcrypt Version 2.5.6 or greater.
Windows users will find the library is the PHP 5.2 Windows binaries release. PHP 5.3 Windows binaries uses the static version of the MCrypt library, no DLL are needed.
If you linked against libmcrypt 2.4.x or higher, the following additional block algorithms are supported: CAST, LOKI97, RIJNDAEL, SAFERPLUS, SERPENT and the following stream ciphers: ENIGMA (crypt), PANAMA, RC4 and WAKE. With libmcrypt 2.4.x or higher another cipher mode is also available; nOFB.