What is the difference between self::$bar and static::$bar in PHP?

What is the difference between using self and static in the example below?

class Foo
{
    protected static $bar = 1234;

    public static function instance()
    {
        echo self::$bar;
        echo "\n";
        echo static::$bar;
    }

}

Foo::instance();

produces

1234
1234

When you use self to refer to a class member, you're referring to the class within which you use the keyword. In this case, your Foo class defines a protected static property called $bar. When you use self in the Foo class to refer to the property, you're referencing the same class.

Therefore if you tried to use self::$bar elsewhere in your Foo class but you had a Bar class with a different value for the property, it would use Foo::$bar instead of Bar::$bar, which may not be what you intend:

class Foo
{
    protected static $bar = 1234;
}

class Bar extends Foo
{
    protected static $bar = 4321;
}

When you call a method via static, you're invoking a feature called late static bindings (introduced in PHP 5.3).

In the above scenario, using self will result in Foo::$bar(1234). And using static will result in Bar::$bar (4321) because with static, the interpreter takes into account the redeclaration within the Bar class during runtime.

// self
var_dump(Foo::$bar);
// (int) 1234

// static
var_dump(Bar::$bar);
// (int) 4321

You typically use late static bindings for methods or even the class itself, rather than properties, as you don't often redeclare properties in subclasses; an example of using the static keyword for invoking a late-bound constructor can be found in this related question: New self vs. new static

However, that doesn't preclude using static with properties as well.