php - Notice: Use of undefined constant ENT_HTML5 - assumed 'ENT_HTML5'

I am trying to create a simple method which accepts the parameters for htmlspecialchars. Although I am getting PHP notice:

Use of undefined constant ENT_HTML5 - assumed 'ENT_HTML5'

1. Any ideas what could be causing this?

/**

 * Encode string.
 *
 * @param array/string $value
 * @param string $param
 * @return string
 */
protected function escape($mixed, $params) {


    $defaults = array('flags' => ENT_QUOTES | ENT_HTML5, 'charset' => 'UTF-8');
    $params = array_merge($defaults, $params);

    if (is_array($mixed)) {
        foreach($mixed as $key => $value) {
            $mixed[$key] = $this->escape($value, $params['flags'], $params['charset']);
        }
    } elseif (is_string($mixed)) {
        $mixed = htmlspecialchars($mixed, $params['flags'], $params['charset']);

    }

    return $mixed;
}

1. If I change: ENT_QUOTES | ENT_HTML5 into: ENT_QUOTES, I get a different error

Warning: htmlspecialchars() expects parameter 2 to be long, string
given

UPDATE

I am using PHP 5.3 so this is the reason for the HTML5 error. If I change ENT_QUOTES | ENT_HTML5 to ENT_COMPAT | ENT_HTML401 I get the same sort of error:

Notice: Use of undefined constant ENT_HTML401 - assumed 'ENT_HTML401'

Answer

ENT_HTML5, ENT_HTML401, and some others were added in PHP version 5.4 according to the manual. For earlier versions those constants are undefined, and PHP will automatically assume that undefined constants are programming "slips" and convert them to strings.