HTTP API how-to?

[phpnewbie112]

I don't know if I used the correct term in the "topic" but I need your help with advices, tips or tutorials that can show how can I create something similar to the following request:

 

let's say I have a database with username,password,balance with a php script that works fine managing users etc...

 

how can I create for ex:

http://domain/script.php?username=user1&password=pwd1&action=getbalance

 

in this way anyone can use this http URL to get the balance from my database using his own script. this also applies to get other actions and tasks...

 

Thanks a million