Zlib is a compression tool from http://zlib.net.
The current version is 188.8.131.52 released Apr 13, 2013 and is VERY stable.
PHP on Windows uses the static version of the library, but can be custom compiled to link to the dll version, simply remove the static version of the library from your LIB path when compiling.
Building for PHP
nmake -f win32/Makefile.msc
This will create 32 or 64 bit zlib_a.lib depending on target used. For the other build options see instructions at the top of win32/Makefile.msc.