I use PHP 5.3.0 and after hours of searching found a
The program can't start because php5.dll is missing from your computer. Try reinstalling the program to fix the problem.
Old question, but are you sure the php_memcache.dll you downloaded is a thread-safe/unsafe compatible with your PHP install?
Look in your PHP directory: if there's a "php5ts.dll" you have a thread-safe PHP, so you need a thread-safe "php_memcache.dll" too, and vice-versa.
I had the same problem, with a diferent php_.dll, and the error message about php5.dll confused the heck out of me for several hours too, but it was just a matter of downloading the right php_.dll.