PHP Singleton Class - Fatal error: Call to private MyObject::__construct() - php

Question

Singleton Class:
<?php
class db_singleton
{
const ORACLE_HOST = "SOMEIP";
const ORACLE_USER = "validuser";
const ORACLE_PASS = "validpass";
const ORACLE_DB = "SOMEIP/DBNAME";
private static $instance; // stores the oci_* instance
private function __construct() { } // block directly instantiating
private function __clone() { } // block cloning of the object
public static function call()
{
// create the instance if it does not exist
if(!isset(self::$instance))
{
// the ORACLE_* constants should be set to or
// replaced with your db connection details
self::$instance = oci_connect(self::ORACLE_USER, self::ORACLE_PASS, self::ORACLE_DB);
if(self::$instance->connect_error)
{
throw new Exception('Oracle connection failed: ' . self::$instance->connect_error);
}
}
// return the instance
return self::$instance;
}
public function __destruct() {
oci_close($instance);
}
public function queryresult($query)
{
$result_set_array =array();
$this->stmt = oci_parse($this->con, $query);
oci_execute($this->stmt);
while($row=oci_fetch_array($this->stmt,OCI_ASSOC+OCI_RETURN_NULLS))
{
$result_set_array[] = $row;
}
oci_free_statement($this->stmt);
return $result_set_array;
}
}
?>
When I try using singleton class with below code, it works perfect and fetch results.
$conn = db_singleton::call();
$stid = oci_parse($conn, 'SELECT * FROM somevalid_table');
oci_execute($stid);
while($result=oci_fetch_array($stid,OCI_ASSOC+OCI_RETURN_NULLS))
{
$result_set_array[] = $result;
}
Now, when I try extending my class using model, it throws exception
class Myclass Extends db_singleton{
public function someModel()
{
$result = parent::queryresult(" select * from somevalid_table");
return $result;
}
}
Exception:
Fatal error: Call to private db_singleton::__construct() from context 'someController'
I know that class cannot be instantiated having private constructor. __construct() functions are always called when an object is instantiated, so trying to do something like $x = new MyObject() will cause a fatal error with a private construction function.
I am using Singleton classes to prevent direct instantiation of an object. How can I overcome issue ? What would be the best solution ?
Thanks.

Answer 1

$x = new MyObject() will never work if your constructor is private in that class because __construct() is the first method that is invoked on object creation.
Create a public method
/**
* Singleton class
*
*/
final class UserFactory
{
/**
* Call this method to get singleton
*
* #return UserFactory
*/
public static function Instance()
{
static $inst = null;
if ($inst === null) {
$inst = new UserFactory();
}
return $inst;
}
/**
* Private ctor so nobody else can instance it
*
*/
private function __construct()
{
}
}
To use:
$fact = UserFactory::Instance();
$fact2 = UserFactory::Instance();
$fact == $fact2;
But:
$fact = new UserFactory()

Answer 2

PHP Class Script:
class db_singleton
{
const ORACLE_USER = "validuser";
const ORACLE_PASS = "validpass";
const ORACLE_DB = "SOMEIP/DBNAME";
private static $instance = null; // stores the oci_* instance
// private constructor
private function __construct() { } // block directly instantiating
private function __clone() { trigger_error('Clone is not allowed.', E_USER_ERROR); } // block cloning of the object
public static function getInstance()
{
// create the instance if it does not exist
if(!isset(self::$instance))
{
// the ORACLE_* constants should be set to or
// replaced with your db connection details
self::$instance = oci_connect(self::ORACLE_USER, self::ORACLE_PASS, self::ORACLE_DB);
if(self::$instance->connect_error)
{
throw new Exception('Oracle connection failed: ' . self::$instance->connect_error);
}
}
// return the instance
return self::$instance;
}
public static function queryresult($query)
{
$result_set_array =array();
$stmt = oci_parse(db_singleton::getInstance(), $query);
oci_execute($stmt);
while($row=oci_fetch_array($stmt,OCI_ASSOC+OCI_RETURN_NULLS))
{
$result_set_array[] = $row;
}
oci_free_statement($stmt);
return $result_set_array;
}
Now to prevent Fatal error: Call to private db_singleton::__construct(), I have added an empty constructor to my child class which is model class in my case. This will override the parent class constructor which is private.
class Myclass Extends db_singleton{
public function __construct() {}
public function someModel(){
$result = parent::queryresult(" select * from somevalid_table");
return $result;
}
}
Hope it helps someone.
Thanks.