The extension_dir is the php system path into which the thirdparty installations were installing the modules like eaccelerator , memcache , etc,. Sometimes this path will be missing from server php.ini . So then you need to find out the correct extension_dir and update it in php.ini . So that the installed extension will work fine.
So the question is how to find the extension_dir . There will be a php-config file with every php installations . You can find it from this file . To find it easily run the following command in shell
# grep extension_dir= `which php-config`
The above result will show an out put like the following ,
# grep extension_dir= `which php-config`extension_dir='/usr/local/lib/php/extensions/no-debug-non-zts-20060613'
#
Please note the path will be different in your server .
Simplified: php -i | grep extension_dir