Compiler functions are called only during compilation of the template.
They are useful for injecting PHP code or time-sensitive static
content into the template. If there is both a compiler function and a
custom function registered under the same name, the compiler function
has precedence.
The compiler function is passed two parameters: the tag argument
string - basically, everything from the function name until the ending
delimiter, and the Smarty object. It's supposed to return the PHP code
to be injected into the compiled template.