What is exactly --enable-zend-multibyte option for when you ./configure PHP?
It is to be able to detect if PHP scripts are Unicode/Multibyte encoded or not (UTF-8 always works and does not need this setting).
I've found this information in this article http://devzone.zend.com/article/2693 (archived copy, the article itself is outdated).
The configuration option --enable-zend-multibyte is part of the zend scripting engine and used for auto-detection of Unicode encoded scripts and files. It looks for BOM (byte order marks) in the begining of the file and uses that to figure out the encoding.