Is it efficient? well according to the original author you take a minor hit it performance (as of yet he has not provided any solid numbers) in exchange for programmer happiness.

The implementation of anonymous functions is quite clever. Please consider this library was created 4 years ago and has remained stagnant up until now. To put it shortly he uses a eval based approach in order for use of anonymous functions.

while (true) { create_function('', ' return 0; '); }