PDO DB Singleton class

<?php

// in the config file:

define('MYSQL_USER', 'mydb');
define('MYSQL_PASS', 'myUser');
define('MYSQL_DB',   'db_myDB');

define('PDO_CONNECTION', true);

define('PDO_MYSQL_DSN', 'mysql:dbname='.MYSQL_DB.';host=localhost;charset=UTF-8');

// Singleton
class DB{
    private static $_PdoDbInstance;

    /*
     * Class Constructor - Create a new database connection if one doesn't exist
     * Set to private so no-one can create a new instance via ' = new DB();'
     */
    private function __construct() {}

    /*
     * Like the constructor, we make __clone private so nobody can clone the instance
     */
    private function __clone() {}

    /*
     * Returns DB instance or create initial connection
     * @param
     * @return $_PdoDbInstance;
     */
    public static function getDB( ) {
        if ( !isset(self::$_PdoDbInstance) ){
            $errorString = '';
            if( !defined('PDO_MYSQL_DSN') ) { $errorString .= 'PDO_MYSQL Data Source Name (DSN) hasn\'t been defined!'.PHP_EOL; }
            if( !defined('MYSQL_USER')    ) { $errorString .= 'MySQL username hasn\'t been defined!'.PHP_EOL; }
            if( !defined('MYSQL_PASS')    ) { $errorString .= 'MySQL password hasn\'t been defined!'.PHP_EOL; }
            if( !defined('MYSQL_USER')    ) { $errorString .= 'MySQL username hasn\'t been defined!'.PHP_EOL; }
            if( !empty($errorString)      ) { $errorString .= 'Class: '.__CLASS__; throw new PDOException( $errorString ); }

            // if no errors have occurred, we create an instance of the PDO database object
            self::$_PdoDbInstance = new PDO( PDO_MYSQL_DSN, MYSQL_USER, MYSQL_PASS, array( PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8' ) );
            // it should throw exceptions in case of any errors
            self::$_PdoDbInstance->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        }
        return self::$_PdoDbInstance;

    }

    /*
     * Release database connection by setting it to NULL
     * @param none
     * @return none
     */
    public static function setNull(){
        self::$_PdoDbInstance = NULL;
    }
}


/////////////////////
// USING THE CLASS //
/////////////////////

try {
    $query = 'SELECT * FROM `test_table` WHERE test_data = :test_data_stuff ';

    $stmt = DB::getDB()->prepare($query);

    $test_data_var = 'blabla';

    $stmt->bindParam( ':test_data_stuff', $test_data_var );

    $stmt->execute();

    $assoc_array = $stmt->fetchAll(PDO::FETCH_ASSOC);

    echo '.<pre>';
    print_r($assoc_array);
    echo '</pre>';
} catch (PDOException $e) {
    DB::setNull();
    echo 'Database error blah-blah...<br />';
    // exception messages should rather be logged!!
    echo $e->getMessage();
} catch (Exception $e) {
    DB::setNull();
    echo 'Any other exceptions blah-blah...<br />';
    // exception messages should rather be logged!!
    echo $e->getMessage();
}

// THAT'S ALL!

?>