database - PHP - a DB abstraction layer use static class vs singleton object?

Question

I don't want to create a discussion about singleton better than static or better than global, etc. I read dozens of questions about similar subjects on SO, but I couldn't come up with an answer to this SPECIFIC question, so I hope someone could now illuminate me by answering this question with one (or more) real simple EXAMPLES, and not just theoretical discussions.

In my app I have the typical DB class to abstract the DB layer and to perform tasks on DB without having to write everywhere in code mysql_connect / mysql_select_db / mysql...

I could write the class either as a STATIC CLASS:

class DB
{
   private static $connection = FALSE; //connection to be opened

   //DB connection values
   private static $server = NULL; private static $usr = NULL; private static $psw = NULL; private static $name = NULL;

   public static function init($db_server, $db_usr, $db_psw, $db_name)
   {
      //simply stores connections values, without opening connection
   }

   public static function query($query_string)
   {
      //performs query over alerady opened connection, if not open, it opens connection 1st
   }

   ...
}

OR as a SINGLETON:

class DBSingleton
{
   private $inst = NULL;
   private $connection = FALSE; //connection to be opened

   //DB connection values
   private $server = NULL; private $usr = NULL; private $psw = NULL; private $name = NULL;

   public static function getInstance($db_server, $db_usr, $db_psw, $db_name)
   {
      //simply stores connections values, without opening connection

      if($inst === NULL)
         $this->inst = new DBSingleton();
      return $this->inst;
   }
   private __construct()...

   public function query($query_string)
   {
      //performs query over already opened connection, if connection is not open, it opens connection 1st
   }

   ...
}

Then after in my app if I want to query the DB i could do

//Performing query using static DB object
DB:init(HOST, USR, PSW, DB_NAME);
DB::query("SELECT...");

//Performing query using DB singleton
$temp = DBSingleton::getInstance(HOST, USR, PSW, DB_NAME);
$temp->query("SELECT...");

To me Singleton has got the only advantage to avoid declaring as static each method of the class. I'm sure some of you could give me an EXAMPLE of real advantage of singleton in this specific case. Thanks in advance.

Answer 1

What is wrong with the following (simplified) example:

class Database
{
    protected $_connection;

    protected $_config;

    public function __construct( array $config ) // or other means of passing config vars
    {
        $this->_config = $config;
    }

    public function query( $query )
    {
        // use lazy loading getter
        return $this->_getConnection()->query( $query );
    }

    protected function _getConnection()
    {
        // lazy load connection
        if( $this->_connection === null )
        {
            $dsn = /* create valid dsn string from $this->_config */;

            try
            {
                $this->_connection = new PDO( $dsn, $this->_config[ 'username' ], $this->_config[ 'password' ] );
            }
            catch( PDOException $e )
            {
                /* handle failed connecting */
            }
        }

        return $this->_connection;
    }
}

$db1 = new Database( array(
    'driver'   => 'mysql',
    'host'     => 'localhost',
    'dbname'   => 'test',
    'username' => 'test_root',
    'password' => '**********'
) );

$db2 = new Database( array(
    'driver'   => 'pgsql',
    'host'     => '213.222.1.43',
    'dbname'   => 'otherdb',
    'username' => 'otherdb_root',
    'password' => '**********'
) );

$someModel       = new SomeModel( $db1 );
$someOtherModel  = new SomeOtherModel( $db2 );
$yetAnotherModel = new YetAnotherModel( $db2 );

This demonstrates how you can make use of lazy loading connections, and still have flexibility to use different database connections.

The database instances will only connect to their individual connection when an object that consumes one of the instances (in this case one of the models) decides to call a method of the instance.