Fix PHP 8.1 deprecations

Author: derrabus

src/HTML5/Parser/DOMTreeBuilder.php

@@ -175,7 +175,7 @@ public function __construct($isFragment = false, array $options = array())
             // documents, and attempting to up-convert any older DTDs to HTML5.
             $dt = $impl->createDocumentType('html');
             // $this->doc = \DOMImplementation::createDocument(NULL, 'html', $dt);
-            $this->doc = $impl->createDocument(null, null, $dt);
+            $this->doc = $impl->createDocument(null, '', $dt);
             $this->doc->encoding = !empty($options['encoding']) ? $options['encoding'] : 'UTF-8';
         }
 
@@ -406,6 +406,8 @@ public function startTag($name, $attributes = array(), $selfClosing = false)
                 $aName = Elements::normalizeMathMlAttribute($aName);
             }
 
+            $aVal = (string) $aVal;
+
             try {
                 $prefix = ($pos = strpos($aName, ':')) ? substr($aName, 0, $pos) : false;