// inside your callback function:
static $ancestors = null;
$parent = (int) $comment->comment_parent;
// Modify INITIAL VALUE here:
$init_val = (int) 1;
$is_child = false;
if ( $parent > (int) 0 )
{
$is_child = true;
$ancestors[] = $parent;
$child_counter = count( $ancestors ) + $init_val;
}
/*
* Only needed in case you want to reset the counter if the current comment has no children
else
{
$ancestors = $child_counter = null;
}
*/
// This shows the number of the current reply counter
if ( $is_child )
printf( __( 'This is the child number: ', TEXTDOMAIN ), $child_counter );