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   name = $n;        $this->age = $a;    }    function intro()    {        return "my name is {$this->name}, i am {$this->age} years old.
";    }    function get_name()    {        return $this->name;    }    function get_age()    {        return $this->age;    }}class student extends people{}$stu = new student('wangfu',21);if($stu instanceof student){    echo "stu is a student class.
";}if($stu instanceof people){    echo "stu is a people class.
";}