I want to add an if statement to my header file

I want to add an id statement in my header template so that I can make it display a subtitle depending on which page (custom static page) it is displaying…

I think I need something like: (although this doesn’t work)

<?php
if(get_permalink(id) == 18 )
{
	print "<h1>hello world!!!</h1>";
}
?>

Can someone help?