title: call_user_func() expects parameter 1 to be a valid callback
body: <p>I'm just playing around with the call_user_func function in PHP and am getting this error when running this simple code:</p>
  
  <pre><code>&lt;?php
  
  
  class A
  {
  
      public $var;
      private function printHi()
      {
  
          echo "Hello";   
  
      }
  
      public function __construct($string)
      {
          $this-&gt;var = $string;   
  
  
      }
  
      public function foo()
      {
  
          call_user_func($this-&gt;var); 
  
      }
  
  }
  
  $a = new A('printHi');
  $a-&gt;foo();
  
  
  ?&gt;
  </code></pre>
  
  <p>I know that if I make a function outside the class called printHi, it works fine, but I'm referring to the class's print hi and not sure why the "this" isn't being registered.</p>