#!/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;
    $cb = function(...$args) use ($cb) { return call_user_func_array($cb,
 $args); }; 
    $cb->bindTo($this); 
    return $cb($baz); 
  }
}

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