If you have decided that you want to build your own PHP extension for your customers to use and include it in the PHP Selector, you might want to do it for each and every supported version of PHP that you have installed. While you don’t have to do that, by installing it for all versions will remove questions that might come up from someone who wants to switch from one version of PHP to another.

The PHP module installation process for alt-php is the same as you would do for standard PHP installation. The only difference is that you have to explicitly use correct version of phpize. Like for PHP 5.2, you would use alt-phpize command /opt/alt/php52/usr/bin/phpize instead of system one, and after compilation finished:
– add ini files for modules to /opt/alt/php52/etc/php.d.all
– add so files to /opt/alt/php52/usr/lib/php/modules
– execute cagefsctl –setup-cl-selector

And do the same for every php version this module should be available.

This information was kindly provided by cloudlinux.