想问下这段代码是什么含义呀?

<?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;
}

}