Optimizing PHP
There are a couple of different things that you can do to improve PHP performance.
Install PHP as ISAPI instead of CGI.
Minimize the amount of extensions loaded.
Zend Optimizer
Free performance booster for PHP
Also runs Zend Guard encoded scripts
http://www.zend.com/products/zend_optimizer/
Tips
If you do not run Zend Guard encoded scripts
You can disable the loader by setting this in your php.ini
zend_optimizer.enable_loader = 0
If you do not use the licesning option you can disable it in your php.ini
zend_optimizer.disable_licensing = 0
Both the above settings will improve Zend Optimizer's performace a little bit.
PHP OpCode Cache Programs
There are a couple of caching programs that work for PHP on Windows. You will need to disable Zend Optimizer if you want to run them.
Advantage of using the cahce programs is it stores the optimized PHP code in a cache for later use while Zend Optimizer optimizes when it runs and throws out the optimized version when its finished.
APC - Alternative PHP Cache
http://us3.php.net/apc/
eAccelerator
http://eaccelerator.net/
Memcache optimizing MySQL performance
Memcache is easy to setup and helps improve MySQL performance. Requires your php scripts to use memcache functions.
Many of the big sites out there including facebook make use of memcache. Memcache can be distributed among many servers.
For information on setuping and installing MemCache read our guide found at
Setup MemCache on IIS