In PHP 5.3:
zend_extension=/local/php/lib/php/extensions/no-debug-non-zts-20090626/xdebug.so
zend_extension=/local/php/lib/php/extensions/debug-zts-20090626/xdebug.so
zend_extension=/local/php/lib/php/extensions/debug-non-zts-20090626/xdebug.so