Doctrine\MongoDB\Query\Expr::wrapEqualityCriteria PHP Method

    private function wrapEqualityCriteria()
    {
        /* If the current field has no criteria yet, do nothing. This ensures
         * that we do not inadvertently inject {"$in": null} into the query.
         */
        if ($this->currentField && !isset($this->query[$this->currentField]) && !array_key_exists($this->currentField, $this->query)) {
            return;
        }
        if ($this->currentField) {
            $query =& $this->query[$this->currentField];
        } else {
            $query =& $this->query;
        }
        /* If the query is an empty array, we'll assume that the user has not
         * specified criteria. Otherwise, check if the array includes a query
         * operator (checking the first key is sufficient). If neither of these
         * conditions are met, we'll wrap the query value with $in.
         */
        if (is_array($query) && (empty($query) || strpos(key($query), '$') === 0)) {
            return;
        }
        $query = ['$in' => [$query]];
    }