We tried ASP.NET for a period of time. However, the flexibility of Zend PHP, the cost savings since it is an open source, and the large open community led us to choose Zend PHP.

Before PHP, I personally used a variety of .NET framework languages. The main reasons I switched were:

• Greater opportunities: There are endless options when it comes to custom web app development. If you can imagine it, it's almost certainly possible with PHP. 
• Large community: If we need assistance or opinions from other experts, there's a large and diverse community ready and willing to help.
• Cost-effectiveness: PHP is fully open source and free to use.
• Access to ready-made solutions: When we start a new project, we're able to easily repurpose predefined classes, libraries, and even entire PHP-based platforms like WordPress. This frees us up to do more on the support side, including migration-related tasks and so on.

We started using Zend while customizing an open source web app developed with Zend, for a customer.