#!/usr/local/bin/php
<?php

final class Foo
{
  private $coef = 1;
  public function __construct() { $this->coef = 2; }
  private $cb = [Foo::class, 'test'];
  private function test($bar) { return $bar * $this->coef; }
  public function wat($baz) { $cb = $this->cb; return $cb($baz); }
}

echo (new Foo)->wat(7);