phpseclib is designed to be ultra-compatible. It works on PHP4+ (PHP4, assuming the use of PHP_Compat) and doesn't require any extensions. For purposes of speed, mcrypt is used if it's available as is gmp or bcmath (in that order), but they are not required.
phpseclib is designed to be fully interoperable with OpenSSL and other standardized cryptography programs and protocols.
This library is written using the same conventions that libraries in the PHP Extension and Application Repository (PEAR) used to be written in (current requirements break PHP4 compatibility). In particular, this library needs to be in your include_path:
<?php set_include_path(get_include_path() . PATH_SEPARATOR . 'phpseclib'); include('Net/SSH2.php'); ?>