title: PHP validation booleans using filter_var
body: <p>I'm using <code>filter_var</code> to validate boolean values but I did not expect it to not recognize <code>FALSE</code>. Why does this happen?</p>
  
  <p><code>filter_var(FALSE, FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE)</code></p>
  
  <p>returns</p>
  
  <p><code>null</code></p>