PHP is newbie-friendly, its convenient to use now that pretty much every webhost provides PHP/Apache server. There are lots of existing libraries and third party scripts for you to integrate without reinventing the wheel. Also the vast number of PHP programmers and journals/articles available on the internet means that you'd never walk alone with PHP.
Python is much less popular, Google App used to only support Python but then decided to add PHP support, you know what this implies.
ASP.NET is a very good alternative actually, while it only works on Windows platform(though its possible to get it work on Linux, its a bit tricky). Since it is static-typed, I'd say that a lot more people would rather go with dynamically typed languages when it comes down to web development.
Ruby is a very difficult programming language to learn and to get used to, you need to learn at least 1-2 programming languages to go easy on it. Even if you are a great programmer yourself it may be necessary to consider whether your co-workers/teammates are up for the task of writing a good ruby program.