php - Array class property

This code gives the error "unexpected '.', expecting ')'". Why is this invalid? I'd thought that as both parts are constant, I could concatenate them. New to PHP. Thanks.

class c {
  const HELLO = 'hello';
  public $arr = array(
    'hw' => self::HELLO . 'world'
  );
}

Answer

Class properties must have constant initial values. The concatenation of those two strings is NOT a constant value.

From the documentation:

[Property] 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.

You could put the property initialisation in your constructor:

public function __construct()
{
  $this->arr = array(
    'hw' => self::HELLO . 'world'
  );
}