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 be moved to DRIVE:\usr\mibs, where DRIVE must be replaced with the driveletter where PHP is installed on, e.g.c:\usr\mibs. Alternatively you may set MIBDIRS environment variable pointed to your MIBs folder.
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.