PHP 5.3.0, 5.3.1 and 5.3.2 do not have SNMP support. SNMP support has been restored in PHP 5.3.3.
The Windows distribution of Net-SNMP contains support files for SNMP in the mibs directory. This directory should added to Windows' environment variables, as MIBDIRS, with the value being the full path to the mibs directory: e.g. c:\usr\mibs.
Important notes for PHP prior to 5.4.0: In order to use the UCD SNMP package, you need to define NO_ZEROLENGTH_COMMUNITY to 1 before compiling it. After configuring UCD SNMP, edit config.h or acconfig.h and search for NO_ZEROLENGTH_COMMUNITY. Uncomment the #define line. It should look like this afterwards:
#define NO_ZEROLENGTH_COMMUNITY 1
If you see strange segmentation faults in combination with SNMP commands, you did not follow the above instructions. If you do not want to recompile UCD SNMP, you can compile PHP with the --enable-ucd-snmp-hack switch which will work around the misfeature.