title: Check existence of elements in a class's array constants in PHP 5.6
body: <p>How can I check that a constant element like A\B::X['Y']['Z'] is set?</p>
  
  <pre><code>&lt;?php
  
  namespace A;
  
  class B
  {
      const X = [
          'Y' =&gt; [
              'Z' =&gt; 'value'
          ]
      ];
  }
  
  var_dump(defined('\A\B::X') &amp;&amp; isset(\A\B::X['Y']['Z']));
  </code></pre>
  
  <p><code>Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in [...] on line 13</code></p>