Constant expression contains invalid operations [duplicate]

Question

This question already has an answer here:

• PHP Error : Fatal error: Constant expression contains invalid operations 4 answers

I have the following code, where I get the error "PHP Fatal Error: Constant expression contains invalid operations". It works fine when I define the variable in the constructor. I am using Laravel framework.

<?php

namespace App;

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

    public function __construct()
    {
    }

}

Answer 1

As described 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.

The only way you can make this work is :-

<?php

namespace App;

class Amazon
{
  protected $serviceURL;

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

Answer 2

Initializing class properties is not allowed this way. You must move the initialization into the constructor.

Answer 3

Another working alternative I used is with boot( ) with Laravel Eloquent:

<?php

namespace App;

class Amazon {
    protected $serviceURL;

    protected static function boot()
    {
        parent::boot();

        static::creating(function ($model){
            $model->serviceURL = config('api.amazon.service_url');
        });
    } }