What type of server-side language you should learn really comes down to preference.
You can achieve your goal using any of them.
Some quick points are..
PHP
- free and constantly being updated by a large community
- large user base makes it easier to find help
- more difficult to find a job due to the competitiveness
- can easily integrate different types of databases
ASP.net
- expensive
- Works only using windows based machines
- lots of large companies still use asp (maybe costs to much to switch) which means it's easier to find a job
Coldfusion
Never used
