There is a mistake in the note by egingell at sisna dot com 12 years ago. The tow dimensional array will output "d,e,f", not "a,b,c".

<?php
$stack = array('a', 'b', 'c');
array_push($stack, array('d', 'e', 'f'));
print_r($stack);
?>

The above will output this:
Array (
  [0] => a
  [1] => b
  [2] => c
  [3] => Array (
     [0] => d
     [1] => e
     [2] => f
  )
)