What is the difference between self and static in PHP?

The main differences is that static allows late static bindings. One of the most useful scenarios that I found was for creating Base classes for Singleton Classes:

class A { // Base Class
    protected static $name = '';
    protected static function getName() {
        return static::$name;
    }
}
class B extends A {
    protected static $name = 'MyCustomNameB';
}
class C extends A {
    protected static $name = 'MyCustomNameC';
}
echo B::getName(); // MyCustomNameB
echo C::getName(); // MyCustomNameC

Using return static::$name in the Base class will return what was statically attached when it was extended.
If you were to use return self::$name then B::getName() would return an empty string as that is what is declared in the Base class.