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PHP Tutorial - Advanced Class Concepts

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Singleton Pattern

The singleton pattern applies to those situations in which there needs to be a single instance of a class. Implementing this pattern makes this single instance easily accessible by many other objects.

Example

<?php

class Example

{

// Hold an instance of the class

private static $instance;

// A private constructor; prevents direct creation of object

private function __construct()

{

echo 'I am constructed';

}

// The singleton method

public static function singleton()

{

if (!isset(self::$instance)) {

$c = __CLASS__;

self::$instance = new $c;

}

return self::$instance;

}

// Example method

public function bark()

{

echo 'Woof!';

}

// Prevent users to clone the instance

public function __clone()

{

trigger_error('Clone is not allowed.', E_USER_ERROR);

}

Magic Methods

The function names _construct, _sleep, _wakeup, _tostring are all magical methods in PHP. There cannot be any functions with these names in the classes. These functions can be had only if we want the magical functionality associated with them.

_sleep

The _sleep magic method is used to close any database connections that the object may have. It is also used to commit pending data and perform similar cleanup tasks. It is also used if there are very large objects which do not have to be saved completely.

_wakeup

The _wakeup magic method is used to reconstruct any resources that the object may have. The basic function of _wakeup is to reestablish any database connections that may have been lost during serialization and perform other initialization tasks.

Example of sleep and wakeup

<?php

class Conn {

protected $link;

private $server, $username, $password, $db;

public function __construct($server, $username, $password, $db)

{

$this->server = $server;

$this->username = $username;

$this->password = $password;

$this->db = $db;

$this->connect();

}

private function connect()

{

$this->link = mysql_connect($this->server, $this->username, $this->password);

mysql_select_db($this->db, $this->link);

}

public function __sleep()

{

mysql_close($this->link);

}

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