Using PHP Code in Templates?

[Martaigne]

I wrote a bit of code that checks if the user is logged in or not. It's basically the same thing behind what you see at the top of the forum mainpage where it either displays the login/register form, or Welcome back, Dorkface!

I'm trying to use this code inside the header template so I can cause the REGISTER button at the top of the page to disappear when a registered user is logged in, and another button to appear instead.

When I inserted the code and tested it, I got some VERY strange results... editing the code various ways caused different strange results, but not what I was expecting. Part of the PHP code would be printed out instead, the table cell would show up above the row of menu items, and the background would be neon green, etc.

I've programmed in PHP before but haven't played with templates yet. How can I utilize code in the them?

Thanks!