Phalcon\Db\Adapter\MongoDB\Functions::isFirstKeyOperator PHP Method

    public static function isFirstKeyOperator($document)
    {
        if (is_object($document)) {
            $document = get_object_vars($document);
        }
        if (!is_array($document)) {
            throw InvalidArgumentException::invalidType('$document', $document, 'array or object');
        }
        $firstKey = (string) key($document);
        return isset($firstKey[0]) && $firstKey[0] == '$';
    }

 /**
  * Constructs a findAndModify command for updating a document.
  *
  * Supported options:
  *
  *  * bypassDocumentValidation (boolean): If true, allows the write to opt
  *    out of document level validation.
  *
  *  * maxTimeMS (integer): The maximum amount of time to allow the query to
  *    run.
  *
  *  * projection (document): Limits the fields to return for the matching
  *    document.
  *
  *  * returnDocument (enum): Whether to return the document before or after
  *    the update is applied. Must be either
  *    FindOneAndUpdate::RETURN_DOCUMENT_BEFORE or
  *    FindOneAndUpdate::RETURN_DOCUMENT_AFTER. The default is
  *    FindOneAndUpdate::RETURN_DOCUMENT_BEFORE.
  *
  *  * sort (document): Determines which document the operation modifies if
  *    the query selects multiple documents.
  *
  *  * upsert (boolean): When true, a new document is created if no document
  *    matches the query. The default is false.
  *
  *  * writeConcern (MongoDB\Driver\WriteConcern): Write concern. This option
  *    is only supported for server versions >= 3.2.
  *
  * @param string       $databaseName Database name
  * @param string       $collectionName Collection name
  * @param array|object $filter Query by which to filter documents
  * @param array|object $update Update to apply to the matched document
  * @param array        $options Command options
  *
  * @throws InvalidArgumentException
  */
 public function __construct($databaseName, $collectionName, $filter, $update, array $options = [])
 {
     if (!is_array($filter) && !is_object($filter)) {
         throw InvalidArgumentException::invalidType('$filter', $filter, 'array or object');
     }
     if (!is_array($update) && !is_object($update)) {
         throw InvalidArgumentException::invalidType('$update', $update, 'array or object');
     }
     if (!Functions::isFirstKeyOperator($update)) {
         throw new InvalidArgumentException('First key in $update argument is not an update operator');
     }
     $options += ['returnDocument' => self::RETURN_DOCUMENT_BEFORE, 'upsert' => false];
     if (isset($options['projection']) && !is_array($options['projection']) && !is_object($options['projection'])) {
         throw InvalidArgumentException::invalidType('"projection" option', $options['projection'], 'array or object');
     }
     if (!is_integer($options['returnDocument'])) {
         throw InvalidArgumentException::invalidType('"returnDocument" option', $options['returnDocument'], 'integer');
     }
     if ($options['returnDocument'] !== self::RETURN_DOCUMENT_AFTER && $options['returnDocument'] !== self::RETURN_DOCUMENT_BEFORE) {
         throw new InvalidArgumentException('Invalid value for "returnDocument" option: ' . $options['returnDocument']);
     }
     if (isset($options['projection'])) {
         $options['fields'] = $options['projection'];
     }
     $options['new'] = $options['returnDocument'] === self::RETURN_DOCUMENT_AFTER;
     unset($options['projection'], $options['returnDocument']);
     $this->findAndModify = new FindAndModify($databaseName, $collectionName, ['query' => $filter, 'update' => $update] + $options);
 }