This is my 2 cents.

First of all, you need understand the market you are in... are you in Nepal or USA or some other country and what language is popular in your area. You haven't mentioned where you are targeting for... Secondly, I wouldn't see much different in PHP or ASP.NET from just web prospective. But if you goto ASP.NET route then you have opportunity of whole .NET framework and then you can also to towards some core development with services or windows application or batch programs or graphics etc... PHP is mostly for web development.

If you are in USA then USA market is dominated by either .NET or Java... I haven't seen many big corporates using PHP... However, popular site like Facebook is written in PHP and MySQL.... so, PHP is big and powerful to handle anything... Good part of PHP is free but for ASP.NET you need to buy softwares from Microsoft....

Regarding security, I have been most of the big banks of USA are using either .NET or Java... I haven't seen any big Bank using PHP (I might be wrong).... so, seems like security is pretty good with ASP.NET... Recently, I have noticed Chase bank moved their online service to ASP.NET from Java....

So, it depends what you want to do with your career... So, what I would say is, if you just want to learn some web programming then go with PHP but if you are looking for career and job in future then either go with .NET or JAVA.... Again, learning PHP is not bad idea, in fact learning anything is not bad....