Howtoforge posted a tutorial about setting up ISPConfig 3.0.5 with support for multiple PHP versions on Ubuntu 12.04 LTS

Since ISPConfig 3.0.5, it is possible to use multiple PHP versions on one server and select the optimal PHP version for a website. This feature works with PHP-FPM (starting with PHP 5.3) and FastCGI (all PHP 5.x versions). This tutorial shows how to build PHP 5.3 and PHP 5.4 as a PHP-FPM and a FastCGI version on an Ubuntu 12.04 server. These PHP versions can be used together with the default PHP (installed through apt) in ISPConfig.

  How To Use Multiple PHP Versions (PHP-FPM/FastCGI) With ISPConfig 3 Ubuntu 12.04 LTS