Description:
------------
Returning composite value (class or array) from tag callback generates circular 
reference on first child item if[!] source value is a composite value 
(see example)

Probably improper work with php variables on extension level. 

Test script:
---------------
<?php
$data=
"#yaml
---
data: !mytag 
  - look upper
...
";

function callback($value){
   return array('data'=>$value,'another'=>'test');
}
$cnt;
$array=yaml_parse($data,0,$cnt,array('!mytag'=>'callback'));

print_r($array['data']);
?>

Expected result:
----------------
Array
(
    [data] => Array
    (
        [0] => look upper
    )
    [another] => test
)

Actual result:
--------------
Array
(
    [data] => Array
 *RECURSION*
    [another] => test
)