"Hello, World!" is not displaying on my page?

Let´s check the obvious first. Have you saved your file, refreshed the page in the browser and as a last resort tried clearing the browser cache and refreshing again? Oh and you're doing this in workspaces correct? If you're doing it locally you need to set up a php server.

Whats happening though? Can you say in more details pelase? Does the page goes all blank? or soemthing? Could you show us the code?

The correct code for outputting hello world would be:

<?php
echo "hello world";

You can try running your code in a PHP sandbox (eg http://sandbox.onlinephpfunctions.com/)to verify correctness. Otherwise, we need to know the details of how your code is not working (environment, whether you're accessing the right page etc.) As Konrad Pilch notes, i did omit the closing tag, and i don't doubt there are points at which doing so is not advised. I'd also note that online sandboxs aren't fully featured, but should be okay for a hello world program, or debugging small fragments of PHP code.

Simon, thats partially correct - depending on the situation.

The code should open PHP tags, and close, in between the code. What you have, it would break my whole aplication where im trying to display user username. Its good for things like classes because thers no html, only PHP, so you don't need to close the PHP tag, and it's better not to.