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
)
)