This is a quick HowTo for installing the eAccelerator PHP cache/optimizer on a Debian system (should be applicable to other Linux distros as well).

1. Download the latest release from SourceForge.net to your server and extract the sources.
2. Change to the sources directory and type in the following.
   export PHP_PREFIX="/usr"
$PHP_PREFIX/bin/phpize
./configure --enable-eaccelerator=shared --with-php-config=$PHP_PREFIX/bin/php-config

   This should set up the environment and compile configuration correctly.
3. Now compile with make and install with make install (as root) afterwards.
4. To configure eAccelerator you have to edit your PHP config which resides in /etc/php4/apache2/php.ini on my system.
   Add the following lines at the end: extension="eaccelerator.so"
eaccelerator.shm_size="16"
eaccelerator.cache_dir="/tmp/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"
   The file README in the sources directory gives a pretty good explanation of all those parameters. The only one you should probably alter is the shm.size because 16MB of shared memory might be a little bit too conservative.
5. Last step is to create the directory where eAccelerator stores the cached scripts which don’t fit into the shared memory. mkdir /tmp/eaccelerator
chmod 0777 /tmp/eaccelerator

Now you’re done. Further tweaking can be accomplished by altering all the parameters in the php.ini file.