How to call a private variable for concatenation inside a class

Question

Please help me to understand this:


1 <?php
2 class test{
3   private $a = "a";
4   private $b = $a."b";
5   
6   function __construct(){
7     echo $this->b;
8   }
9 }

I get Constant expression contains invalid operations on line 4

Why? I tried also with $this->a but it also do not work.

Please explain it to me.

Update

For some strange reason this works


class test{
  private const a = "a";
  private $b = self::a."b";
    
  function __construct(){
    echo $this->b;
  }
}

I can remember now that it does not work. But can someone please explain it to me.

You can't perform operations in class property declaration. If you want to change the value of `b` to be `a` plus something, do it in the constructor. — El_Vanja, Mar 29 '21 at 15:58
Honestly, it seems like you shouldn't even have the `b` property, only a getter that takes `a` and appends whatever you need. Because if you change `a`, then you also need to change `b` if it's a property. — El_Vanja, Mar 29 '21 at 16:06
After the edit, you concatenate two *constant* expressions. In the first one, you used a variable. — El_Vanja, Mar 29 '21 at 16:16
@El_Vanja so operations with constants are permitted but operations with variables not? — Anylyn Hax, Mar 29 '21 at 16:18
Exactly, because it seems variable values aren't resolved yet at that point during compilation. See [this question](https://stackoverflow.com/questions/40171546/php-error-fatal-error-constant-expression-contains-invalid-operations). — El_Vanja, Mar 29 '21 at 16:23

jspit · Answer 1 · 2021-03-30T07:37:08.823

The property declaration may include an initialization, but this initialization must be a constant value (->PHP-manual). This

private $b = $this->a."b";

throw a 'Fatal error: Constant expression contains invalid operations..' because $this->a is not a constant. The PHP parser cannot check whether $this->a always remains constant. For PHP, $this->a is generally a variable.

That, however, works flawlessly.

class test
{
  private const a = "a";
  private $b = self::a."b";
}

self::a and "b" are constants.

How to call a private variable for concatenation inside a class

1 Answers1