(PHP) Singleton Database Class - what about static methods?

Question

It's a basic website. Based off answers on here, I'm doing this:

private $db;

public function __construct($id = null) {
    $this->db = Db::getInstance(); //singleton from the Db class

But if there is a static method, I can't use the object specific variable.

Is there anything better than having to manually specify the db variable inside the static method?

public static function someFunction($theID){
    $db = Db::getInstance();

EDIT: Making the variable static doesn't solve the problem. Access to undeclared static property. I'd still have to assign the variable within the static function. The question is asking if there's a way around this.

My DB Class (although not important to this discussion):

class Db {
private static $m_pInstance;
private function __construct() { ... }

public static function getInstance(){
    if (!self::$m_pInstance)
        self::$m_pInstance = new Db();
    return self::$m_pInstance;
}

}

Answer 1

Yes, you can make the $db static:

static private $db;

I'm assuming that's what you need, since you're accessing it from a static method. If there's any reason why you wouldn't want this, that must mean that the method probably shouldn't be static.

EDIT:

As per @zerkms (thanks) comments, you access static variables with self:::

self::$db = Db::getInstance(); 

Answer 2

You've outlined one of the issues you come across when co-mingling static methods within a class that can also be insantiated, setting member variables via instantiation, and expecting access to their values through static method call. The only real way around this is to set a class specific singleton for the database singleton, or pass in the DB object to the static method.

// Option 1
class MyClass
{
    private static $db;

    public function __construct($id = null)
    {
        self::$db = Db::getInstance(); //singleton from the Db class
    }

    public static function someFunction($theID)
    {
        self::$db->query('SELECT * FROM my_table');    
    }
}

// Singleton DB for MyClass will be initalized via constructor
$myClass = new MyClass();    

// This call will have access to DB object set via class specific singleton
MyClass::someFunction(4);

// Option 2
class MyClass
{
    private $db;

    public function __construct($id = null)
    {
        $this->$db = Db::getInstance(); //singleton from the Db class

        if (!is_null($id)) {
            $this->id = $id;
        }
    }

    public function getDb()
    {
        return $this->db;
    }

    public function getId()
    {
        return $this->id;
    }

    // Sub-Option 1: If Id ISNT set via object
    public static function someFunction($object, $theID)
    {
        $object->getDb()->query('SELECT * FROM my_table WHERE id = ' . (int) $theID);    
    }

    // Sub-Option 2: If Id IS set via object
    public static function someFunction($object)
    {
        $object->getDb()->query('SELECT * FROM my_table WHERE id = ' . (int) $object->getId());    
    }
}

// Sub-Option 1 call
$myClass = new MyClass();

MyClass::someFunction($myClass, 4);

// Sub-Option 2 call
$myClass = new MyClass(4);

MyClass::someFunction($myclass);