Question

PHP PDO Singleton Class:

<?php

require_once('app/config.php'); // Require constants HOST, DATABASE, USER, PASSWORD

/*

dbConnection class.

Manages connections to and operations on the database. Call dbConnection::getInstance() to return an instance.
Prepare your statements by calling prepareQuery() on the object

Attribute list:

$instance:
> Static self instance to manage database resource

$connection:
> Holds connection resource

$sth:
> Statement handler variable. Handles SQL statements.
_______________________________________________________________________________________________________________

Method list:

getInstance():
> Creates or returns existing connection to the database

prepareQuery():
> Prepares the $sth variable for execution.

bindParameter():
> Binds parameters to the $sth variable.

numRows($query):
> Returns the number of returned from a query

runQuery():
> Executes the current statement on the database

fetchRow():
> Executes the current statement then returns an associative array

fetchObj($className, $parameters = NULL):
> Executes the current statement and returns an object of your specification. Also takes additional parameters to pass to the object's constructor.


*/

class dbConnection
{   
    private static $instance = NULL;
    private $connection;
    private $sth;

    function __construct()
    {
        $this->connection = new PDO('mysql:host=' . HOST . ';dbname=' . DATABASE, USER, PASSWORD);
    }

    function getInstance()
    {
        if (self::$instance == NULL)
            self::$instance = new dbConnection();
        return self::$instance;
    }

    function prepareQuery($query)
    {
        $this->sth = $this->connection->prepare($query);
    }

    function bindParameter($number, $value)
    {
        $this->sth->bindParam($number, $value);
    }

    function numRows()
    {   
        try
        {
            $this->sth->execute();

            $count = $this->sth->rowCount();
            return $count;
        }
        catch(PDOException $e)
        {
            echo __LINE__.$e->getMessage();
        }
    }

    function runQuery()
    {
        try
        {
            $this->sth->execute() or print_r($connection->errorInfo());
        }
        catch(PDOException $e)
        {
            echo __LINE__.$e->getMessage();
        }
    }

    function fetchRow()
    {
        try
        {
            $this->sth->setFetchMode(PDO::FETCH_ASSOC);
            $this->sth->execute();
            return $this->sth;
        }
        catch(PDOException $e)
        {
            echo __LINE__.$e->getMessage();
        }
    }

    function fetchObj($className, $parameters = NULL)
    {
        try
        {
            $this->sth->setFetchMode(PDO::FETCH_CLASS, $className, $parameters);
            $this->sth->execute();
            return $this->sth;
        }
        catch(PDOException $e)
        {
            echo __LINE__.$e->getMessage();
        }
    }
}
?>

How to test singleton?
Take an object at a time, which is closed when you are finished with the object at the end.

Answer 1

I think you are misapplying the Singleton pattern here.

Nevertheless, testing Singletons is possible. Quoting Testing Code that uses Singletons

PHPUnit has a backup/restore mechanism for static attributes of classes.

This is yet another feature of PHPUnit that makes the testing of code that uses global state (which includes, but is not limited to, global and superglobal variables as well as static attributes of classes) easier.

Also see http://www.phpunit.de/manual/current/en/fixtures.html#fixtures.global-state

The @backupStaticAttributes annotation that is discussed in the section called “@backupStaticAttributes” can be used to control the backup and restore operations for static attributes. Alternatively, you can provide a blacklist of static attributes that are to be excluded from the backup and restore operations like this

So if you wanted to disable the backup, you’d do

class MyPdoTest extends PHPUnit_Framework_TestCase
{
    protected $backupStaticAttributesBlacklist = array(
      'dbConnection' => array('instance')
    );    

    // more test code
}

Also have a look at the chapter on Database Testing