const 关键字
const 常量名 = 常量值

class Student
{
public $stu_id;
public $stu_name;
public $stu_gender;
const GENDER_MALE= '男';
const GENDER_FEMALE = '女';
}

class Student
{
public $stu_id;
public $stu_name;
public $stu_gender;
const GENDER_MALE= '男';
const GENDER_FEMALE = '女';
public function __construct($id,$name,$gender='')
{
$this->stu_id= $id;
$this->stu_name= $name;
$this->gender= ($gender == ' ')?self::GENDER_MALE : $gender;
}
}