Ii is quit difficult to say that which is better.
 
PHP and ASP.NET are two of the most widely used languages for development of web applications. Both allow development of complex and sophisticated sites,but ASP.NET provide more security than PHP.

PHP is a language. PHP consists of a platform-independent engine that parses PHP scripts. This language also provides common web application functionality such as database connectivity.

ASP.NET is not a language, but a technology and is a small part of the .NET Framework. The Dot NET Framework consists of a) CLR – Common Language Runtime which manages execution of the code b) a hierarchical set of class libraries. These libraries are extensive and provide a great deal of functionality both for web-based applications and well as windows-based.

ASP.NET framework is built entirely on an OOP paradigm and OOP concepts while PHP is not.

PHP does not follow the OOP paradigm entirely. It only supports partial encapsulation, and partial polymorphism. 


Compilation is also different in PHP and ASP.NET. 

In PHP, when a page is requested, the HTML and inline PHP script is compiled to binary format called Zend Opcodes. Once compiled, the opcodes are run by the Zend Engine and HTML is generated which is sent to the client. Commercial products are available that increase the speed at which PHP pages are executed.

In contrast, when an ASP.NET page is first requested it is converted into an intermediate language called MSIL (Microsoft Intermediate Language). The CLR then converts this MSIL to machine code. Conversion to machine code only takes place once for each page request (unless code has been modified). All subsequent requests use the already generated machine code. In ASP.NET no optimization product is required as the MSIL generated is already in its most optimized form. 

    
    There are various reasons for asp.net is better then PHP.

ASP.net providing robust security then PHP.

Caching concept in more matured then PHP like any type data we can cache in ASP.net.

Support to various programming languages like C#.net , VB.net etc.

ASP.net ASP.net support many patterns and Oops concepts.

ASP.net development environment is very friendly . Its very helpful to programmer for development.

and many more :) 

    
    Hi,

PHP is just and interpreted language though it is strong enough to develop any kind of page you want to develop.
ASP.NET is a part of a framework which means you may use the framework s abilities. Your ASP.NET application
can be a part of a mobile, Windows, Windows Service, Smart client or even unmanaged code supported.
Security and that kind of things are mostly bound to the developers. If you forget escaping and replacing 
some chars when requesting a data using $_REQUEST or $_POST, your PHP application may also be an unsecure 
system.

So depending on the solution that you will develop, both languages have their advantages and disadvantages.

All the best,

Toni