Description:
------------
Missed ability to emit tagged elements on yaml_emit.
see example as possible use-case solution

Test script:
---------------
class SampleClass {
   public $data;    // data may be in any pecl/yaml suitable type
   /**
    * Yaml emit callback function, referred on yaml_emit call by class name
    */
   public static function yamlEmit(SampleClass $obj){
      return array('tag'=>'!sample','data'=>$obj->data);
   }
}

$t=new SampleClass();
$t->data=array ('a','b','c');
$r=yaml_emit(array('our_val'=>$t),YAML_ANY_ENCODING,YAML_ANY_BREAK,
   array('SampleClass'=>array('SampleClass','yamlEmit')));
echo $r;

Expected result:
----------------
---
our_val: !sample
  - a
  - b
  - c