diff --git a/action.php b/action.php
index 3728ca1..f9560f9 100644
--- a/action.php
+++ b/action.php
@@ -11,7 +11,9 @@ function register(Doku_Event_Handler $controller) {
     }
 
     function _renderer_content_postprocess(&$event, $param) {
-        if ($_GET['do'] !== 'export_revealjs' && $this->getConf('revealjs_active')) {
+        global $INPUT;
+
+        if ($INPUT->get->str('do') !== 'export_revealjs' && $this->getConf('revealjs_active')) {
             /* close last edit section correctly (missing </div>), because we close sections
             only when a new one is opened - and this logic fails for the last section in the
             document */
diff --git a/renderer.php b/renderer.php
index 698b87f..5cf6caa 100644
--- a/renderer.php
+++ b/renderer.php
@@ -477,7 +477,7 @@ function _highlight($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($text, -1) == "\n") {
diff --git a/syntax/header.php b/syntax/header.php
index fb30608..c0dbdef 100755
--- a/syntax/header.php
+++ b/syntax/header.php
@@ -47,13 +47,14 @@ public function connectTo($mode) {
     public function handle($match, $state, $pos, Doku_Handler $handler) {
         /* We reuse and adapt here the default DokuWiki header handler code. See also
         /inc/parser/handler.php around line 97. */
+        global $INPUT;
 
         // get level and title
         $title = trim($match);
         $level = 7 - strspn($title,'=');
         if($level < 1) $level = 1;
         $title = trim($title,'= ');
-        if ($this->getConf('revealjs_active') || $_GET['do']=='export_revealjs') {
+        if ($this->getConf('revealjs_active') || $INPUT->get->str('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,