PHP 面向对象 final类与final方法

Fatal error: Class SuperMath may not inherit from final class (Math) in E:PHPProjects est.php教程 on line 14

<?
//声明一个final类Math
class Math{
public static $pi = 3.14;
public function __toString(){
return "这是Math类。";
}
public final function max($a,$b){
return $a > $b ? $a : $b ;
}
}
//声明类SuperMath 继承自 Math类
class SuperMath extends Math {
public final function max($a,$b){}
}
//执行会出错，final方法不能被重写。

?>
<?
//声明一个final类Math
final class Math{
public static $pi = 3.14;

public function __toString(){
return "这是Math类。";
}
}
$math = new Math();
echo $math;

//声明类SuperMath 继承自 Math类
class SuperMath extends Math {
}
//执行会出错，final类不能被继承。

?>

Fatal error: Class SuperMath may not inherit from final class (Math) in E:PHPProjects est.php on line 16