Manual:$wgUseEAccelShm

Details
(1.5) You can use this for persistent caching where your wiki runs on a single server. Enabled by default if eAccelerator is installed. Mutually exclusive with memcached and Turck MMCache, the preference order being memcached first, Turck MMCache second, and eAccelerator third. Most of the code to support this is directly copied from the Turck code.

Installing eAccelerator from SVN
MediaWiki requires that you have eaccelerator shared memory functions (like eaccelerator_get, The one she tests for during installation) wikitech-l/2007-September/033680.html. Assuming you have one PHP installation.

svn --username anonymous --password anonymous co http://dev.eaccelerator.net/eaccelerator/trunk eaccelerator
 * Download and cd eaccelerator


 * Compile and install

phpize sudo ./configure --with-eaccelerator-shared-memory --with-eaccelerator-sessions sudo make sudo make install


 * Configure

Now you need to make PHP aware of your installation, So you need to find and edit/create a configuration file that is readable by PHP (php.ini). In Ubuntu, It is found in /etc/php5/conf.d (anything there will be read), So make/edit a file with the following:

extension="eaccelerator.so" eaccelerator.shm_size="16" eaccelerator.cache_dir="/var/cache/eaccelerator" eaccelerator.enable="1" eaccelerator.optimizer="1" eaccelerator.check_mtime="1" eaccelerator.debug="0" eaccelerator.filter="" eaccelerator.shm_max="0" eaccelerator.shm_ttl="0" eaccelerator.shm_prune_period="0" eaccelerator.shm_only="0" eaccelerator.compress="1" eaccelerator.compress_level="9"

sudo mkdir /var/cache/eaccelerator sudo chown www-data /var/cache/eaccelerator sudo /etc/init.d/apache2 restart

www-data may be different as it is the name of the user Apache runs on.

Now you can choose eAccelerator during MediaWiki installation.

Note
eAccelerator 0.9.5 causes the MediaWiki 1.9.3 installer to crash when run for a second time under PHP 5.2.x, and potentially other problems. The patch at https://www.eaccelerator.net/ticket/231 resolves this issue with version 0.9.5. This issue is already fixed in the upcoming 0.9.6 release.