title: PHP instanceof returns false for true condition
body: <p>I'm thoroughly confused as to why php's <code>instanceof</code> operator insists that the LVALUE here is not an instance of the defined class when <code>get_class</code> says that it is.</p>
  
  <pre><code>class WhereIn {
      public function __construct($args) {
          echo "is instanceof: " . ($args[0] instanceof ActiveRecordField) . EOL;
          echo "get class: " . get_class($args[0]) . EOL;
      }
  }
  </code></pre>
  
  <p>The output from this method is:</p>
  
  <pre><code>is instanceof: 
  get class: ActiveRecordField
  </code></pre>
  
  <p>For reference, I'm using PHP 5.6.9.</p>