How to debug PHP functions with FirePHP when the function definition and invocation are not in the same file?

Question

I have installed FirePHP, and can get output on console with the following example:

<?php
require_once('FirePHPCore/fb.php');  
ob_start(); 

function hello() {
  fb('Hello World!', FirePHP::TRACE);
}
function greet() {
  hello();
}
greet();

?>

But if I move the greet(); to another php file, I can get nothing on console. I have tried to include the require_once & ob_start in both files, but it didn't take effect. How to debug PHP functions with FirePHP when the function definition and invocation are not in the same file?