title: The input value of false for FILTER_VALIDATE_BOOLEAN
body: <p>I want the input to accpet 'true' or 'false' only, so I am trying to use <code>FILTER_VALIDATE_BOOLEAN</code> to do that,</p>
  
  <pre><code>if (!filter_var('false', FILTER_VALIDATE_BOOLEAN)) 
  {
      $error = true;
      echo 'error';
  }
  </code></pre>
  
  <p>It echoes the error message but it should not.</p>
  
  <p>How can I make it right?</p>