call static instead of self on protected method

Author: Franck

src/PhpWord/Shared/Html.php

@@ -61,7 +61,7 @@ public static function addHtml($element, $html, $fullHTML = false, $preserveWhit
          * @todo parse $stylesheet for default styles.  Should result in an array based on id, class and element,
          * which could be applied when such an element occurs in the parseNode function.
          */
-        self::$options = $options;
+        static::$options = $options;
 
         // Preprocess: remove all line ends, decode HTML entity,
         // fix ampersand and angle brackets and add body tag for HTML fragments
@@ -82,10 +82,10 @@ public static function addHtml($element, $html, $fullHTML = false, $preserveWhit
         $dom = new DOMDocument();
         $dom->preserveWhiteSpace = $preserveWhiteSpace;
         $dom->loadXML($html);
-        self::$xpath = new DOMXPath($dom);
+        static::$xpath = new DOMXPath($dom);
         $node = $dom->getElementsByTagName('body');
 
-        self::parseNode($node->item(0), $element);
+        static::parseNode($node->item(0), $element);
         if (\PHP_VERSION_ID < 80000) {
             libxml_disable_entity_loader($orignalLibEntityLoader);
         }