Why PHP? It is easily the most popular programming language on the web today that is cross platform, meaning it can run on windows or linux. Steering clear of proprietary programming variants makes us more viable as a company because we support a broader range of clients.
We actually started life as a ASP.NET shop and migrated to PHP due to overwhelming demand from our clients. When you develop solely in .NET you essentially become unmarketable to the more than half of your potential clients, clients that will only allow unix/linux servers in their organization. Most industries that value security like financial services, health care, government, etc. required us to program in PHP, hence the change. While both have their advantages only PHP is cross platform.
What do WordPress, Joomla, Magento, Drupal and phpBB have in common? Well, not only are they hands down the most popular applications for blogs, content management, and ecommerce systems on the web today, but they are also written in PHP. Amazing things happen when you offer open-source solutions and have ginormous online communities contributing to them. While most of our clients require a custom hand coded solution many can take advantage of the cost savings provided by open source applications like the wildly popular wordpress app.