(PECL mongo >= 0.9.0)
MongoDBRef::isRef — Checks if an array is a database reference
   This method does not actually follow the reference, so it does not determine if it is
   broken or not.  It merely checks that ref is in valid 
   database reference format (in that it is an object or array with $ref and $id
   fields).
  
ref
     Array or object to check.
    Returns TRUE on success or FALSE on failure.