Set A variable from other Variable in a class without function - php

Question

I started using classes in PHP, and I have the following code:
class webConfig
{
public $SALT1 = "3525";
public $SALT2 = "2341";
public $SALT3 = $SALT1.$SALT2;
}
But this gives me 500 Internal Server Error, which means that I am doing something wrong.
Is there any way of doing this without using any function, including construct?
This does not work aswell:
class webConfig
{
public $SALT1 = "3525";
public $SALT2 = "2341";
public $SALT3 = $this->SALT1.$this->SALT2;
}

Answer 1

In PHP; Expressions are not allowed as Default Values of any Property. For this reason, PHP is simply saying that you should not even imagine doing that in the first place. But, fortunately then; this is why you have the Constructor Method — to help you initialize some class Properties, assign Default values to Member Properties using Expression (like in your case), etc.... Using a Constructor, your class could look like the Snippet below and it is expected to work as well:
<?php
class webConfig {
public $SALT1 = "3525";
public $SALT2 = "2341";
public $SALT3;
public function __construct(){
// SET THE VALUE $SALT3 HERE INSTEAD
$this->SALT3 = $this->SALT1 . $this->SALT2;
}
}
Alternatively; you may even initialize all the Properties of the Class within the Constructor like so:
<?php
class webConfig {
public $SALT1; //<== NOTICE THAT THIS PROPERTY IS NOT INITIALIZED.
public $SALT2; //<== NOTICE THAT THIS PROPERTY IS NOT INITIALIZED.
public $SALT3; //<== NOTICE THAT THIS PROPERTY IS NOT INITIALIZED.
public function __construct(){
$this->SALT1 = "3525";
$this->SALT2 = "2341";
$this->SALT3 = $this->SALT1 . $this->SALT2; //<== YET AGAIN; SET THE VALUE
//<== OF $SALT3 LIKE BEFORE
}
}