Fix Example #4 of filter_var (#4252)

Author: kamil-tekiela

reference/filter/functions/filter-var.xml

@@ -170,14 +170,16 @@ bool(false)
 
   <example>
    <title>Providing flags either directly or via an <type>array</type></title>
-   <programlisting>
+   <programlisting role="php">
 <![CDATA[
+<?php
 
 $str = 'string';
 
 var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE));
 var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN, ['flags' => FILTER_NULL_ON_FAILURE]));
 
+?>
 ]]>
    </programlisting>
    &example.outputs;