Linux knowledge is not necessary to learn php. Easy to use Linux-Apache-Php packages are available for windows also. EasyPhp, wamp server are a few to name.
Advantages of learning php.
1. Easy to learn. If you know C, it will be very easy. It uses the same syntax. All you need to lean is the concept.
2. Php hosting is cheap
3. Popular blog managers like wordpress and CMSs like joomla and drupal are biult on php. If you want to master them in future, select php
Advantages of asp.net
1. You will get better salary, atleast here in india, compared to php
2. You are allowed to code in any of the available languages like vb.net, c#.net, vc++.net etc