When writing a web application in todays environment, the first thing you will have to ask yourself is, PHP or ASP.net? Both are fairly large languages and have alot of backing from their respective communities.
The first thing you need to ask yourself is how much can you afford? If you are not already developing in ASP.net, the initial cost can be quite expensive. Microsoft licensing for Visual Studio, IIS and Microsoft Server don't come cheap. Although they do have free licenses you can start with, if you are developing on an enterprise level, you will quickly outgrow these. Compare that to PHP which in comparison, is absolutely free. You can start from scratch and apart from the cost of the machine to host on, everything is 100% free and in most cases open source.
Opensource is an important part of PHP and is in contrast to alot of what ASP.net is about. PHP relies on a community effort to continue developing the technology and keep the push for using it in order. However this comes with a few downsides in that you are often relying on third party tools to complete a project. In ASP.net, you are relying on one person only, Microsoft. This can be a hinderance as any advances in technology need to come from Microsoft to begin with, PHP on the other hand is very open to change from the community.
Out of the box, PHP does not come with any specific templating structure. You can use the inbuilt "Include" functions to try and cobble together common parts between webpages, but it is no where near as intuitive as ASP.net's "MasterPages". In saying that, PHP does have fairly nice Templating models, but once again it is a third party product not direct with PHP.
The most important thing you have to ask yourself when you start a Web Development project is what is right for the project, not what are you a fan of. Keep to this rule and you should come out fine.
If you want to learn more, you can read PHP vs ASP checklists both here and here.