(PECL mongo >= 1.5.0)
MongoId::isValid — Check if a value is a valid ObjectId
This method may be used to check a variable before passing it as an argument to MongoId::__construct().
value
     The value to check for validity.
   Returns TRUE if value is a
   MongoId instance or a string consisting of exactly 24
   hexadecimal characters; otherwise, FALSE is returned.