Running nginx is a must, now if you run PHP-FPM set the one pool and set it to use few workers. You will see what is the memory usage of each PHP-FPM child, however running a web server on 64 MB machine would need certain drastic drawbacks, as not using MySQL (or disabling the InnoDB storage and optimizing MySQL for this low-level environment). Better yet is to use a remote MySQL database or use SQLite for that matter.

Also, if you set caching in nginx be prepared for it to eat up some memory too, if disable it you should have a web server running happily under 45-50 MB.