未定义的类常量'CONDITION_OPTIONS'

Question

我有一个Bicycle类，其中常量变量protected const CONDITION_OPTIONS，每当我尝试回显来自condition()方法的返回值时，此消息就会出现

Fatal error: Uncaught Error: Undefined class constant 'CONDITION_OPTIONS' in C:\Users\Khaled\Desktop\sites\chain_gang\private\classes\bicycle.class.php:64 Stack trace: #0 C:\Users\Khaled\Desktop\sites\chain_gang\public\bicycles.php(38): Bicycle->condition() #1 {main} thrown in C:\Users\Khaled\Desktop\sites\chain_gang\private\classes\bicycle.class.php on line 64

请检查下面的代码...

<?php

class Bicycle {
      protected $condition_id;
      protected const CONDITION_OPTIONS = [
   1 => 'Beat up',
   2 => 'Decent',
   3 => 'Good',
   4 => 'Great',
   5 => 'Like New'
  ];


  public function __construct($arg=[]){
   $this->condition_id = $arg['condition_id'] ?? 3;
  }


  public function condition(){
   if($this->condition_id > 0){
    return self::CONDITION_OPTIONS[$this->condition_id];
   }else{
    return "Unknown";
   }

  }
}



$arg = ['condition_id' => 2];
$bike = new Bicycle($arg);
echo $bike->condition();
?>