How can I make my class variable assignment dynamic?

Question

In the following code, I want to assign an $settings's key, help to a constant class variable value, $To_default. However, I am told Constant expression contains invalid operations. Is there any way I can get around this?

class EmailClass extends PluginBase {

  private $To_default = 'scrollout@stackoverflow.com';

  protected $settings = array(
    'To'=>array(
      'type'=>'text',
      'label'=>'To',
      'help'=>$this->To_default, // Constant expression contains invalid operations
    ),
  );

I've tried declaring $To_default in various ways including private const $To_default, static private $To_default, etc. but none worked. $settings is not static, as you can see, so I don't understand why this is a problem.

Answer 1

Don't know deep technical explanation for this but I think this is because normally you initialize properties in the constructor, like so:

class EmailClass 
{
private $To_default = 'scrollout@stackoverflow.com'; // not a constant, so better throw it into constructor too
protected $settings;

public function __construct() {
    $this->settings = array(
    'To'=>array(
    'type'=>'text',
    'label'=>'To',
    'help'=>$this->To_default
    )
    );
}
}

Analogical I don't know why: 'if isset(expression)' doesn't work but it doesn't have to. There's a better solution: 'if(expression)'. This is just how we do it.