__set() __get() _isset() __unset() 在__unset() 在类中没有事先声明和已经声明过的属性调用unset的区别

<?php
//echo strtr("I Love Mysql, Love PHP", "Mysql","MYSQL");
//$ar = array('Mysql'=>'MYSQL','PHP'=>'php');
//echo strtr("I Love Mysql, Love PHP", $ar);

/*
class myClass
{

      public static function autoload($className)
      {                 
            if(is_file($className.".class.php"))
            {
            	require_once($className.".class.php");

            }else{
            	echo "{$className}类文件不存在!";
            }   

      }
}
spl_autoload_register(array('myClass','autoload'));
$User = new User();
echo $User->getUserName();
echo "<BR><BR>";*/

class Base
{
	  private $UserName;

	  private $Password;

	  public function __get($key)
      {
            return $this->$key;
      }

      public function __set($key, $val)
      {
      	   echo "key=={$key},val=={$val}<BR><BR>";
      	   $this->$key = $val;
      }


      public function __isset($key)
      {
           echo "<BR>===============call isset=============<BR>";
           return isset($this->$key);
      }


      public function __unset($key)
      {
           echo "<BR>===============call __unset=============<BR>";
           unset($this->$key);

      }
}

$base = new Base();
$base->UserName = "xlc";
$base->Password = "888888";
$base->age = 22;
echo "UserName:{$base->UserName}, Password:{$base->Password},age:{$base->age}<BR><BR>";


echo "<BR><h1>age</h1><BR>";
unset($base->age); //直接在类中没有声明的属性，在unset时，并不会调用魔术方法__unset()
if(isset($base->age)) //但__isset()是进行调用的
{
	echo "存在";
}else{
	echo "不存在";
}
echo "<BR>=======================<BR>";



echo "<BR><h1>Password</h1><BR>";
unset($base->Password);    
if(isset($base->Password))  //如果在类中定义了该属性，这里会调用两次__isset();
{
	echo "存在";
}else{
	echo "不存在";
}

echo "<BR><h1>UserName</h1><BR>";
unset($base->UserName);
if(isset($base->UserName))
{
	echo "存在";
}else{
	echo "不存在";
}
echo "<BR>=======================<BR>";

//echo isset($base->$name) ? "property is setted.<br>" : "property is not setted.<br>"; 
/*$key = "Userame";
//当属性已经存在的情况下就不走__isset()了这是为何?
//这里只有在$key为该对像已经定义的属性时才会调用__isset()魔术方法
if(isset($base->$key))
{
	echo "属性存在";
}else{
	echo "属性不存在";
}*/

?>