* PHP stands for PHP: Hypertext Preprocessor
* PHP is a server-side scripting language, like ASP
* PHP scripts are executed on the server
* PHP supports many databases (MySQL, Informix, Oracle, Sybase, Solid, PostgreSQL, Generic ODBC, etc.)
* PHP is an open source software
* PHP is free to download and use
My favorite when it comes to the web: PHP5.
But then, all languages are good if you know how to use them.
I suppose the biggest concern here is security. (we are talking about money^^)
If we talk about security, then PHP is NOT the reference.
Unless you follow very strict security criterias, any competent hacker will be able to play with your code. (seen it with my own eyes!)
This is not (or much less) the case with compiled languages.
As a good example, I'me still waiting for a bank application fully written in php.
You must also be aware of your responsability as programer. If you write your own payment system, you will be fully responsible for all transactions. On the other hand, there are ready made unexpensive payment systems that you may use that will guarantee you a minimum of quality and take that financial responsability of your back.
Anyway, whatever language you choose, i wish you success,
I think you should not much concentrate on language, but still guys prefer php, since there are many ready-made api's available to integrate. It lowers down cost of development, easy to develop. Even I have experienced search engines love it. However every programming language get rendered into html but it is light weighted.
If you use ASP.NET server cost, development cost gets higher than these open source programming language. But at last but not the least its all about functionality; If php fulfills your requirement you should opt else choose which serve your purpose.