Nesting functions in PHP

Question

Simple PHP question:

Why does this work,

$exclude_exts = array('js', 'css',);
$filename = "test.css";
$ext = explode('.',$filename);
$is_excluded = in_array(strtolower(array_pop($ext)), $exclude_exts);

but this doesn't.

$exclude_exts = array('js', 'css',);
$filename = "test.css";
$is_excluded = in_array(strtolower(array_pop(explode('.',$filename))), $exclude_exts);

Edit: Both used to work in a previous version of PHP (I forgot which version).

Answer 1

Because array_pop requires a reference, since it alters the array in place. When you pass the return value of explode there is no variable there to reference.