PHPの継承に親クラスを導入する必要はありますか?

class MyClass  
{ 
 public $dat = 0; 
 public function __construct($dat) { 
  $this->dat = $dat; 
 } 
 public function getDat() { 
  return "$this->dat\n"; 
 } 
} 
class MySubClass extends MyClass 
{ 
 public function getDat() { 
  return "dat: $this->dat\n"; 
 } 
} 
$a = new MyClass(3); 
$b = new MySubClass(4); 
echo $a->getDat();  // 3 
echo $b->getDat();  // dat: 4

class MyClass  
{ 
 private $name = ""; 
 public $num = 0; 
 public $str = ""; 
 public function __construct($name) { 
  $this->name = $name; 
  $this->num = 100; 
  $this->str = "none"; 
 } 
 public function getName() { 
  return $this->name; 
 } 
} 
class MySubClass extends MyClass 
{ 
 public function __construct($name, $str) { 
  parent::__construct($name);    // 调用父类方法 
  $this->num = "0"; 
  $this->str = $str; 
  echo parent::getName()."\n";    // 调用父类方法 
 } 
 public function getName() { 
  return parent::getName()."$this->str\n"; // 调用父类方法 
 } 
} 
$b = new MySubClass("myName", true);  // myName 
echo $b->getName();          // myName1 
class MyClass  
{ 
 final public function getName() { 
 } 
}

class MyClass  
{ 
 public $a = 1; 
 protected $b = 2; 
 private $c = 3; 
 public function f1() { 
  echo "MyClass f1\n"; 
  echo "\$a:$this->a; \$b:$this->b; \$c:$this->c;\n"; 
 } 
 protected function f2() { 
  echo "MyClass f2\n"; 
  echo "\$a:$this->a; \$b:$this->b; \$c:$this->c;\n"; 
 } 
 private function f3() { 
  echo "MyClass f3\n"; 
 } 
} 
class MySubClass extends MyClass  
{ 
 public $b = 22; 
 public $c = 33; 
 public function f1() { 
  echo "MySubClass f1\n"; 
  // 继承到父类中的$a属性，直接使用 
  echo "\$a:$this->a; \$b:$this->b; \$c:$this->c;\n"; 
  // 调用父类中的同名方法 
  parent::f1(); 
  // 继承到父类中的f2()方法，直接使用 
  $this->f2(); 
 } 
 // 父类的f3()是私有的，这里的定义与父类无关 
 public function f3() { 
  echo "MySubClass f3\n"; 
 } 
} 
$b = new MySubClass; 
$b->f1();echo "\n"; 
/* 
MySubClass f1 
$a:1; $b:22; $c:33; 
MyClass f1 
$a:1; $b:22; $c:3; 
MyClass f2 
$a:1; $b:22; $c:3; 
*/ 
$b->f3();echo "\n"; 
/* 
MySubClass f3 
*/

// 声明为final的类不能被继承 
final class MyClass 
{ 
 private $dat; 
 public function __construct($dat) { 
  $this->dat = $dat; 
 } 
 // final方法不能被覆盖，不过此类已经是final类，方法无必要在声明为final了 
 final public function getDat() { 
  return $this->dat; 
 } 
}