title: Check if array value isset and is null
body: <p>How to check if array variable</p>
  
  <pre><code>$a = array('a'=&gt;1, 'c'=&gt;null);
  </code></pre>
  
  <p>is set and is null.</p>
  
  <pre><code>function check($array, $key)
  {
      if (isset($array[$key])) {
          if (is_null($array[$key])) {
              echo $key . ' is null';
          }
          echo $key . ' is set';
      }
  }
  
  check($a, 'a');
  check($a, 'b');
  check($a, 'c');
  </code></pre>
  
  <p>Is it possible in PHP to have function which will check if $a['c'] is null and if $a['b'] exist without "PHP Notice: ..." errors?</p>