class A 
{
 public static $_a = 'Class A';

 public static function echoProperty()
 {
 echo self::$_a . PHP_EOL;
 }
}

class B extends A 
{
 public static $_a = 'Class B';
}

$obj = new B();
B::echoProperty();//输出 Class A

class A 
{
 public static $_a = 'Class A';

 public static function echoProperty()
 {
 echo static::$_a . PHP_EOL;
 }
}
//再次调用B::echoProperty将输出 'CLASS B'

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_self()); // A
echo get_class(A::get_static()); // A