What is the scope of require_once in PHP?

Simple question: Is the scope of require_once global?

For example:

<?PHP

require_once('baz.php');

// do some stuff
foo ($bar);

function foo($bar) {
    require_once('baz.php');
    // do different stuff
}

?>

When foo is called, does it re-parse baz.php? Or does it rely on the already required file from the main php file (analagous to calling require_once twice consecutively for the same include file)?

I saw this thread before, but it didn't quite answer the question:

Should require_once "some file.php" ; appear anywhere but the top of the file?

Thanks for your help!