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(PECL mongo >= 1.5.0)
MongoId::isValid — Check if a value is a valid ObjectId
$value 
   )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.