What are the major advantages of using ASP versus PHP

Not a great deal...
Although, ASP is getting old. ASP.NET is one up from ASP, and if you're going to start learning a new language you should choose ASP.NET over ASP because ASP is being phased out.
There are a few advantages of ASP.NET over PHP...
- It's a Microsoft technology so support is available for it from Microsoft, whereas if you have a major issue with PHP your best bet is to post a message on a forum and hope somebody knows the answer. Not great when your business critical e-commerce website goes down and you're loosing money for every minute of downtime.
- It's a compiled language. I don't know what the speed difference is like with PHP (probably not a lot in it, it depends on the server anyway) but apparently it kicks J2EE's butt.
- It's far more structured (code wise) that both ASP and PHP. Because ASP and PHP are scripted, they are tightly integrated within the HTML code. with ASP.NET, you put easily identifyable controls into the HTML, and reference them from a seperate code file.
- Because it's based on .NET, it's very extensible and there's lots of room for code re-use.

For more info on ASP.NET, see http://www.asp.net

PHP is petite. ASP.NET is bloated.
PHP - It just works. Simply fast and cool.

Everything is available online for PHP. You can learn PHP very easily rather than ASP.NET.

Microsoft has some cool tools for .NET. You can build a website without knowing any coding. DotNetNuke portal is available as opensource. You can try that also.

For PHP, there are so many content management systems
PhpNuke, Joomla, Xoops etc.,
It is a matter of choice for you. Pick one. ;-)