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There are many tutorials out there covering how to install APCu from command line (as root user), but none on how to install APCu on a WHM/cPanel server " the easy way ". And it's actually pretty easy provided your server is executing PHP in FastCGI mode (the most optimal setup). Default cPanel installations usually run PHP in suPHP mode, which does not allow APCu to be installed. If you have full control of the server, you can use EasyApache in WHM to "rebuild" your system with PHP in FastCGI mode.

FastCGI mode is known to be a fast and secure way to run PHP on your server... FastCGI is also preferred compared to mod_php as it allows websites to execute with the user's permissions (like with suPHP) and as some claim, it's also the fastest from all other PHP builds.

If you want to re-build your server with FastCGI, you can grab a ready-made profile to upload and use in WHM: http://engintron.com/files/cpanel/engintron.dtd

Don't bother trying to install APCu via cPanel's PECL modules installer. It's just broken.

Now, assuming you have PHP in FastCGI mode running on your WHM/cPanel (version 11.48+) based server, here are the steps to install APCu on your server:

1. Install packages

Login via SSH as "root" user on your cPanel server and execute the following commands:

$ yum install make pcre-devel
$ pecl install channel://pecl.php.net/APCu-4.0.10

The first line installs some required packages. Year 2014 or later builds of cPanel should have these installed. If you run the command and the packages are already installed, nothing will happen. The second line will retrieve the latest 4.x build of APCu directly from the PECL code repositories which is compatible with (at the time of writing) the PHP versions (5.x) that ship with cPanel. APCu's v5.x is currently compatible with PHP 7.x only so don't install that until cPanel includes this PHP version too.

Restart Apache via WHM

2. Edit php.ini

Now edit the master php.ini file of the server - usually located in /usr/local/lib/php.ini - to add a couple of configuration options for APCu. Open the file (either with some editor like Coda or via "nano" or "vim") and append these lines at the very bottom of the php.ini file:

[apcu]
apc.enabled = 1
apc.shm_size = 128M

The second line will create a memory pool of 128MBs of cache available from the system to APCu. If you have lots of sites that will use APCu for caching, adjust accordingly.

Restart Apache via WHM.

3. Adjust settings for APCu

If you wish to adjust the APCu values above, you can now do so via "PHP Configuration Editor" in WHM, after selecting the "Advanced Mode" editing option. These 2 values will be at the very top. APCu has a lot more options but "apc.shm_size" is obviously the most critical as it defines the memory pool to hold all cache objects coming from your CMS (Joomla, WordPress etc.).

Remember that whenever your edit your PHP's configuration, you need to restart Apache for the changes to take effect.

4. Keep in mind

If you rebuild your cPanel's LAMP stack with EasyApache, you need to re-install APCu as your changes will be lost. Just follow steps 1-3 once more.

What next?

Wanna go the extra mile and do more to improve your cPanel server's performance, besides the FastCGI setup and APCu? You can obviously optimize your Apache, FastCGI & MySQL configurations:

• For Apache (make sure you run EasyApache and check Apache 2.4 - it's way better than Apache 2.2), you can have a look at Linode's knowledge base as a starting point https://www.linode.com/docs/websites/apache-tips-and-tricks/tuning-your-apache-server or better still, try using ApacheBuddy to get insights on your existing configuration: https://github.com/gusmaskowitz/apachebuddy.pl
  
  To give you an idea, this is a good starting point:
  
  StartServers 5
  MinSpareServers 5
  MaxSpareServers 10
  ServerLimit 500
  MaxRequestWorkers 500 (this option was previously called MaxClients in Apache v2.2)
  MaxRequestsPerChild 4000
  Timeout 20
  
  In cPanel, these Apache directives can be configured in "Apache Configuration" (in WHM).
• For MySQL have a look at this bulletproof configuration we created with the help of Percona (the MySQL experts) a few years ago and updated to today's needs & uses: https://gist.github.com/fevangelou/fb72f36bbe333e059b66 - the file to edit on your server is: /etc/my.cnf
• For FastCGI/PHP have a look here http://2bits.com/articles/apache-fcgid-acceptable-performance-and-better-resource-utilization.html & here https://www.cloudlinux.com/blog/clnews/perfecting-fastcgi-settings-for-shared-hosting.php - the file to edit on your server is: /usr/local/apache/conf/php.conf
  
  After you edit this file, you need to "merge" your changes with cPanel so they don't get reset to the defaults when Apache's configuration is updated. If however you rebuild Apache & PHP via EasyApache, you need to run the entire process as the changes to php.conf are reset.
  
  So after you edit the php.conf file, execute the following commands as root via SSH:
  
  $ /usr/local/cpanel/bin/apache_conf_distiller --update
  $ /scripts/rebuildhttpdconf
  $ service httpd restart
• Use Engintron: http://engintron.com/

(updated Jan 2016)