self - 就是这个类，是代码段里面的这个类。
static - PHP 5.3加进来的只得是当前这个类，有点像$this的意思，从堆内存中提取出来，访问的是当前实例化的那个类，那么 static 代表的就是那个类。
差异应该在于new self是实例化当前代码所在类。new static是调用的的那个类，例如，MyTest继承了Test类, 那么当在MyTest中使用static的时候，这个static代表的就是MyTest，self代表的就是Test。
还是看看老外的专业解释吧：
self refers to the same class whose method the new operation takes place in.
static in PHP 5.3's late static bindings refers to whatever class in the hierarchy which you call the method on.
In the following example, B inherits both methods from A. self is bound to A because it's defined in A's implementation of the first method, whereas static is bound to the called class (also see get_called_class() ).
class A {
public static function get_self() {
return new self();
}
public static function get_static() {
return new static();
}
}
class B extends A {}
echo get_class(B::get_self()); // A
echo get_class(B::get_static()); // B
echo get_class(A::get_static()); // A
如何解决掉 return new static($val); 这个问题呢？
class A {
public function create1() {
$class = get_class($this);
return new $class();
}
public function create2() {
return new static();
}
}
class B extends A {
}
$b = new B();
var_dump(get_class($b->create1()), get_class($b->create2()));
/*
The result
string(1) "B"
string(1) "B"
*/