The Zend Engine in Php

When PHP3 was written, an entirely new parser was created from the ground up.
PHP4 represents a similar change to the scripting engine. This rewrite, though, is
more significant by orders of magnitude.Zend is a scripting engine that sits below the PHP-specific modules. It is optimized
to significantly improve performance.
These changes in efficiency will ensure PHP4's continued success. Most code written
for PHP3 will continue to run with no changes; however, these scripts may run up to
200 times faster!
A commercial addition to the Zend engine will be the facility for compiling PHP
scripts. This will provide a further gain in performance that should leave most, if not
all, competitors far behind.
Zend is built to improve performance but is also designed for increased flexibility.
Communication with servers has been improved, so it will be possible to create PHP
modules that work with a wider range of servers. Unlike a CGI interpreter, which sits
outside a server and is initialized every time a script is run, a server module runs in
conjunction with the server. This improves performance because the scripting
engine does not need to be started for a PHP page to be executed.