Make php8 compatible

fixes neuralyzer#41

action.php

@@ -11,7 +11,7 @@ function register(Doku_Event_Handler $controller) {
     }
 
     function _renderer_content_postprocess(&$event, $param) {
-        if ($_GET['do'] !== 'export_revealjs' && $this->getConf('revealjs_active')) {
+        if (isset($_GET['do']) && $_GET['do'] !== 'export_revealjs' && $this->getConf('revealjs_active')) {
             /* close last edit section correctly (missing </div>), because we close sections
             only when a new one is opened - and this logic fails for the last section in the
             document */

renderer.php

@@ -477,7 +477,7 @@ function _highlight($type, $text, $language = null, $filename = null, $options =
             $this->doc .= '</a></dt>'.DOKU_LF.'<dd>';
         }
 
-        if($text{0} == "\n") {
+        if($text[0] == "\n") {
             $text = substr($text, 1);
         }
         if(substr($text, -1) == "\n") {

syntax/header.php

@@ -53,7 +53,7 @@ public function handle($match, $state, $pos, Doku_Handler $handler) {
         $level = 7 - strspn($title,'=');
         if($level < 1) $level = 1;
         $title = trim($title,'= ');
-        if ($this->getConf('revealjs_active') || $_GET['do']=='export_revealjs') {
+        if ($this->getConf('revealjs_active') || (isset($_GET['do']) && $_GET['do']=='export_revealjs')) {
             /* We are now on a reveal.js activated page and we want to do our
             own section handling to be able to get all relevant content from
             one slide into one edit section. Since sections are header driven,