请教下大神这段代码是什么含义了?

<?php
/** NotORM - simple reading data from the database
* @link http://www.notorm.com/
* @author Jakub Vrana, http://www.vrana.cz/
* @copyright 2010 Jakub Vrana
* @license http://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
* @license http://www.gnu.org/licenses/gpl-2.0.html GNU General Public License, version 2 (one or other)
*/

if (!interface_exists('JsonSerializable')) {
    interface JsonSerializable {
        function jsonSerialize();
    }
}

include_once dirname(__FILE__) . "/NotORM/Structure.php";
include_once dirname(__FILE__) . "/NotORM/Cache.php";
include_once dirname(__FILE__) . "/NotORM/Literal.php";
include_once dirname(__FILE__) . "/NotORM/Result.php";
include_once dirname(__FILE__) . "/NotORM/MultiResult.php";
include_once dirname(__FILE__) . "/NotORM/Row.php";

// friend visibility emulation
abstract class NotORM_Abstract {
    protected $connection, $driver, $structure, $cache;
    protected $notORM, $table, $primary, $rows, $referenced = array();

    protected $debug = false;
    protected $debugTimer;
    protected $freeze = false;
    protected $rowClass = 'NotORM_Row';
    protected $jsonAsArray = false;
    protected $isKeepPrimaryKeyIndex = FALSE; //@dogstar 20151230

    protected function access($key, $delete = false) {
    }

}

/** Database representation
* @property-write mixed $debug = false Enable debugging queries, true for error_log($query), callback($query, $parameters) otherwise
* @property-write bool $freeze = false Disable persistence
* @property-write string $rowClass = 'NotORM_Row' Class used for created objects
* @property-write bool $jsonAsArray = false Use array instead of object in Result JSON serialization
* @property-write string $transaction Assign 'BEGIN', 'COMMIT' or 'ROLLBACK' to start or stop transaction
*/
class NotORM extends NotORM_Abstract {

    /** Create database representation
    * @param PDO
    * @param NotORM_Structure or null for new NotORM_Structure_Convention
    * @param NotORM_Cache or null for no cache
    */
    function __construct(PDO $connection, NotORM_Structure $structure = null, NotORM_Cache $cache = null) {
        $this->connection = $connection;
        $this->driver = $connection->getAttribute(PDO::ATTR_DRIVER_NAME);
        if (!isset($structure)) {
            $structure = new NotORM_Structure_Convention;
        }
        $this->structure = $structure;
        $this->cache = $cache;
    }

    /** Get table data to use as $db->table[1]
    * @param string
    * @return NotORM_Result
    */
    function __get($table) {
        return new NotORM_Result($this->structure->getReferencingTable($table, ''), $this, true);
    }

    /** Set write-only properties
    * @return null
    */
    function __set($name, $value) {
        if ($name == "debug" || $name == "debugTimer" || $name == "freeze" || $name == "rowClass" || $name == "jsonAsArray" || $name == 'isKeepPrimaryKeyIndex') {
            $this->$name = $value;
        }
        if ($name == "transaction") {
            switch (strtoupper($value)) {
                case "BEGIN": return $this->connection->beginTransaction();
                case "COMMIT": return $this->connection->commit();
                case "ROLLBACK": return $this->connection->rollback();
            }
        }
    }

    /** Get table data
    * @param string
    * @param array (["condition"[, array("value")]]) passed to NotORM_Result::where()
    * @return NotORM_Result
    */
    function __call($table, array $where) {
        $return = new NotORM_Result($this->structure->getReferencingTable($table, ''), $this);
        if ($where) {
            call_user_func_array(array($return, 'where'), $where);
        }
        return $return;
    }

}