title: Can I test if a callback is valid?
body: <p>I'd like to be able to test to see whether or not a callback is valid before I try to call it. Is this possible?</p>
  
  <p>If I call <code>call_user_func</code> or <code>call_user_func_array</code> with something like <code>array($this, 'methodThatDoesNotExist')</code> PHP warns with <code>[E_WARNING] call_user_func() expects parameter 1 to be a valid callback</code>.</p>