Server Side Scripting PHP & IIS?

i been through alot over the past few days trying to configure PHP and IIS.Heres my question.Why is it that I have to do all this configuration inorder to see a PHP script I wrote,when I can go on the internet and see PHP webpages without configing anything at all? My second question is if I put my website on the internet, how can I be sure everyone can view my website? I know everyone isn't gonna have IIS or PHP. Is there something I need to do to my code? Is there something I need to upload to the online site's directory? I'm so confused.