I'm using XAMPP as my local development environment on Snow Leopard. The XAMPP package does not include APC (Alternative PHP Cache). If we simply do a PECL install, you'll find that PHP complains about the APC architecture. This happens because the XAMPP for OS X is 32 bit, and Snow Leopard is 64 bit, hence PECL builds a 64 bit apc.so.
To overcome this issue, we need to manually compile APC from source and here is how I did it.
First, download APC source from http://pecl.php.net/package/APC and double click on the tar file within finder will unzip it.
cd ~/Downloads/APC-3.1.3p1/APC-3.1.3p1/ phpize MACOSX_DEPLOYMENT_TARGET=10.6 CFLAGS="-arch i386 -g -Os -pipe -no-cpp-precomp" CCFLAGS="-arch i386 -g -Os -pipe" CXXFLAGS="-arch i386 -g -Os -pipe" LDFLAGS="-arch i386 -bind_at_load" ./configure --enable-apc --with-apxs --with-php-config=/Applications/XAMPP/xamppfiles/bin/php-config-5.3.1 make sudo cp modules/apc.so /Applications/XAMPP/xamppfiles/lib/php/php-5.3.1/extensions/no-debug-non-zts-20090626/.
We should see no errors and we should have the new APC so file copied into our PHP extension directory.
Next, enable APC from php.ini by adding the following 2 lines
extension=apc.so apc.shm_size = 32
Now if you do php -m from command line, you'll see APC listed as a loaded module.
Hope it helps and save you from pulling your hair out.