PHP5.1 issue with the way array_push used in book_next

The following code works with PHP4

array_push($path = book_location($node), $node); // Path to top-level node including this one.

but not PHP5.1 --- $path ends up containing only the book location. Changing this to

$path = book_location($node);
array_push($path, $node);

makes book_next work as expected. (Sorry: I could not verify if PHP5.0 exhibits the same issue.)