新人学习PHP问题，发现书上代码不对

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><?php	class Person	{		private $name;		private $sex;		private $age;				function __construct($name="",$sex="男",$age=23)		{			$this->name=$name;			$this->sex=$sex;			$this->age=$age;		}		function say()		{			echo "我的名字  ".$this->name."，性别  ".$this->sex."，年龄  ".$this->age."<br>";		}		function run()		{			echo $this->name."正在走路<br>";		}	}	class Student extends Person 	{		var $school;		function study()		{			echo $this->name."正在".$this->school."学习<br>";		}	}	class Teacher extends Student 	{		var $wage;		function teaching()		{			echo  $this->sex."正在".$this->school."教学，每个月工资为".$this->wage."<br>";		}	}	$teacher1=new Teacher("张三","男",40);	$teacher1->school="沈阳工学院";	$teacher1->wage=3000;		$teacher1->say();	$teacher1->run();	$teacher1->teaching();?>