php - 常量表达式包含无效操作

Question

我有以下代码，其中出现错误“PHP fatal error :常量表达式包含无效操作”。当我在构造函数中定义变量时，它工作得很好。我正在使用 Laravel 框架。

<?php

namespace App;

class Amazon
{
    protected $serviceURL = config('api.amazon.service_url');

    public function __construct()
    {
    }

}

我看到过这个问题:PHP Error : Fatal error: Constant expression contains invalid operations 但我的代码没有将任何内容声明为静态，因此这没有回答我的问题。

Answer 1

如上所述here

Class member variables are called "properties". You may also see them referred to using other terms such as "attributes" or "fields", but for the purposes of this reference we will use "properties". They are defined by using one of the keywords public, protected, or private, followed by a normal variable declaration. This declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

完成这项工作的唯一方法是:-

<?php

namespace App;

class Amazon
{
  protected $serviceURL;

  public function __construct()
  {
    $this->serviceURL = config('api.amazon.service_url');
  }
}