libiconv is a character set conversion library from http://ftp.gnu.org/pub/gnu/libiconv/
PHP currently uses 1.14 released 2011.08.07
libiconv development has been moved to https://savannah.gnu.org/projects/libiconv/
For PHP suitable sources and up-to-date information see https://github.com/winlibs/libiconv.
Because iconv.exe is GPL licensed, the author has decided to drop support for building on msvc altogether, because building on newer compilers that require separate distributions of MS runtimes might cause gpl violations (you can simply point people to the runtime installers provided by microsoft and not include them with the program...but that's another story). However, libiconv is lgpl and does not have these restrictions, the versions on php.net are compiled with Microsoft compilers with a small patch to fix compiler warnings and errors.
If someone has the time or energy the patch should probably be sent upstream to the bugs list.
Building for PHP
The solution has static and dll projects. Both of them have to be built in debug and release mode. The binaries have to be put into the deps tree with the following structure:
lib: libiconv_debug.lib libiconv_a_debug.lib libiconv_a.lib libiconv.lib
bin: libiconv_debug.pdb libiconv_debug.dll libiconv.pdb libiconv.dll
After this the iconv PHP extension can be built static or shared.