diff --git a/ODT/ODTFootnote.php b/ODT/ODTFootnote.php
index cd9ab53..0fc3bb0 100644
--- a/ODT/ODTFootnote.php
+++ b/ODT/ODTFootnote.php
@@ -20,7 +20,7 @@ class ODTFootnote
      *
      * @author Andreas Gohr <andi@splitbrain.org>
      */
-    function footnoteOpen(ODTInternalParams $params, $element=NULL, $attributes=NULL) {
+    static function footnoteOpen(ODTInternalParams $params, $element=NULL, $attributes=NULL) {
         // $element and $attributes are actually unused
 
         // Move current content to store and record footnote
@@ -40,7 +40,7 @@ function footnoteOpen(ODTInternalParams $params, $element=NULL, $attributes=NULL
      *
      * @author Andreas Gohr
      */
-    function footnoteClose(ODTInternalParams $params) {
+    static function footnoteClose(ODTInternalParams $params) {
         // Close any open paragraph first
         $params->document->paragraphClose();
 
diff --git a/ODT/ODTIndex.php b/ODT/ODTIndex.php
index 7bd2cf1..3c3adc3 100644
--- a/ODT/ODTIndex.php
+++ b/ODT/ODTIndex.php
@@ -333,9 +333,17 @@ protected static function getTOCBody(array $toc, $p_styles, $stylesLNames, $max_
             // Only add the heading to the TOC if its <= $max_outline_level
             if ( $params [3] <= $max_outline_level ) {
                 $level = $params [3];
-                $content .= '<text:p text:style-name="'.$p_styles [$level].'">';
+                $styleName = "";
+                if (isset($p_styles[$level])) {
+                    $styleName = $p_styles[$level];
+                }
+                $content .= '<text:p text:style-name="'.$styleName.'">';
                 if ( $links == true ) {
-                    $content .= '<text:a xlink:type="simple" xlink:href="#'.$params [0].'" text:style-name="'.$stylesLNames [$level].'" text:visited-style-name="'.$stylesLNames [$level].'">';
+                    $styleLName = "";
+                    if (isset($stylesLNames[$level])) {
+                        $styleLName = $stylesLNames[$level];
+                    }
+                    $content .= '<text:a xlink:type="simple" xlink:href="#'.$params [0].'" text:style-name="'.$styleLName.'" text:visited-style-name="'.$styleLName.'">';
                 }
                 $content .= $params [2];
                 $content .= '<text:tab/>';
diff --git a/ODT/ODTTable.php b/ODT/ODTTable.php
index 70348c0..7e2347a 100644
--- a/ODT/ODTTable.php
+++ b/ODT/ODTTable.php
@@ -21,7 +21,7 @@ public static function tableOpen(ODTInternalParams $params, $maxcols = NULL, $nu
         if (!isset($element)) {
             $element = 'table';
         }
-        $elementObj = $params->elementObj;
+        $elementObj = isset($params->elementObj) ? $params->elementObj : null;
 
         // Close any open paragraph.
         $params->document->paragraphClose();
@@ -398,7 +398,7 @@ public static function tableOpenUseCSS(ODTInternalParams $params, $maxcols=NULL,
      * @param null $numrows
      */
     public static function tableOpenUseProperties (ODTInternalParams $params, $properties, $maxcols = 0, $numrows = 0){
-        $elementObj = $params->elementObj;
+        $elementObj = isset($params->elementObj) ? $params->elementObj : null;
 
         // Eventually adjust table width.
         if ( !empty ($properties ['width']) ) {
@@ -426,7 +426,10 @@ public static function tableOpenUseProperties (ODTInternalParams $params, $prope
      */
     public static function tableAddColumnUseProperties (ODTInternalParams $params, array $properties = NULL){
         // Add column and set/query assigned style name
-        $styleName = $properties ['style-name'];
+        $styleName = null;
+        if (array_key_exists('style-name', $properties)) {
+            $styleName = $properties['style-name'];
+        }
         $styleNameGet = '';
         self::tableAddColumn ($params, $styleName, $styleNameGet);
 
diff --git a/ODT/ODTUtility.php b/ODT/ODTUtility.php
index 75fcfbf..f299aa2 100644
--- a/ODT/ODTUtility.php
+++ b/ODT/ODTUtility.php
@@ -196,10 +196,10 @@ public static function deleteUselessElements(&$docContent, array $preventDeletet
     public static function getImageSize($src, $maxwidth=NULL, $maxheight=NULL){
         if (file_exists($src)) {
             $info  = getimagesize($src);
-            if(!$width){
+            if(!isset($width)){
                 $width  = $info[0];
                 $height = $info[1];
-            }else{
+            } else {
                 $height = round(($width * $info[1]) / $info[0]);
             }
 
@@ -434,7 +434,7 @@ public static function adjustValueForODT ($property, $value, ODTUnits $units) {
 
                 // If it is a short color value (#xxx) then convert it to long value (#xxxxxx)
                 // (ODT does not support the short form)
-                if ( $part [0] == '#' && $length == 4 ) {
+                if (isset($part[0]) && $part[0] == '#' && $length == 4 ) {
                     $part = '#'.$part [1].$part [1].$part [2].$part [2].$part [3].$part [3];
                 } else {
                     // If it is a CSS color name, get it's real color value
diff --git a/ODT/css/cssimportnew.php b/ODT/css/cssimportnew.php
index 5686f43..c952a21 100644
--- a/ODT/css/cssimportnew.php
+++ b/ODT/css/cssimportnew.php
@@ -73,7 +73,7 @@ public function matches (array $attributes=NULL) {
             switch ($this->operator) {
                 case '=':
                     // Attribute should have exactly the value $this->value
-                    if ($attributes [$this->attribute] == $this->value) {
+                    if (isset($attributes[$this->attribute]) && $attributes[$this->attribute] == $this->value) {
                         return true;
                     } else {
                         return false;
@@ -206,6 +206,7 @@ public function __construct($simple_selector_string) {
 
         $content = '';
         $first_sign = '';
+        $next_sign = '';
         $first = true;
         $pseudo_element = false;
         while ($pos < $max) {
@@ -1259,7 +1260,7 @@ protected function inherit (array &$dest, array $parents) {
         // (MUST be done backwards!)
         $max = count ($parents);
         foreach ($parents as $parent) {
-            $properties = $parent->getProperties ();
+            $properties = $parent->getProperties() ?? [];
             foreach ($properties as $key => $value) {
                 if ($dest [$key] == 'inherit') {
                     $dest [$key] = $value;
diff --git a/ODT/elements/ODTElementTable.php b/ODT/elements/ODTElementTable.php
index 91c8cb2..0be7366 100644
--- a/ODT/elements/ODTElementTable.php
+++ b/ODT/elements/ODTElementTable.php
@@ -62,7 +62,7 @@ public function getOpeningTag () {
         if (!isset($style_name)) {
             $encoded = '<table:table>';
         } else {
-            $encoded .= '<table:table table:style-name="'.$style_name.'">';
+            $encoded = '<table:table table:style-name="'.$style_name.'">';
         }
         $maxcols = $this->getTableMaxColumns();
         $count = $this->getCount();
@@ -171,7 +171,7 @@ public function getTableColumnStyles() {
      * @param array $value
      */
     public function getTableColumnStyleName($column) {
-        return $this->table_column_styles [$column];
+        return $this->table_column_styles [$column] ?? null;
     }
 
     /**
@@ -331,12 +331,15 @@ public function getMaxWidthOfNestedContainer (ODTInternalParams $params, array $
         } else if ($cell_style->getProperty('padding') != NULL) {
             $value = $cell_style->getProperty('padding');
             $value = $params->document->toPoints($value, 'y');
-            $padding += 2 * $value;
+            if (is_numeric($value)) {
+                $padding += 2 * $value;
+            }
         }
 
         $table_column_styles = $this->getTableColumnStyles();
         $style_name = $table_column_styles [$column-1];
         $style_obj = $params->document->getStyle($style_name);
+        $width = 0;
         if (isset($style_obj)) {
             $width = $style_obj->getProperty('column-width');
             $width = trim ($width, 'pt');
@@ -425,6 +428,7 @@ public function adjustWidth (ODTInternalParams $params, $allowNested=false) {
         $width = $this->adjustWidthInternal ($params, $max_width);
 
         $style_obj = $params->document->getStyle($table_style_name);
+        $rel_width = 0;
         if (isset($style_obj)) {
             $style_obj->setProperty('width', $width.'pt');
             if (!$this->isNested ()) {
@@ -436,7 +440,7 @@ public function adjustWidth (ODTInternalParams $params, $allowNested=false) {
                 }
             } else {
                 // Calculate rel width in relation to maximum table width
-                if ($max_width != 0) {
+                if (is_numeric($max_width) && $max_width != 0) {
                     $rel_width = round(($width * 100)/$max_width);
                 }
             }
@@ -466,7 +470,7 @@ public function adjustWidthInternal (ODTInternalParams $params, $maxWidth) {
         $table_column_styles = $this->getTableColumnStyles();
         $replace = true;
         for ($index = 0 ; $index < $this->getTableMaxColumns() ; $index++ ) {
-            $style_name = $table_column_styles [$index];
+            $style_name = $table_column_styles [$index] ?? null;
             $style_obj = $params->document->getStyle($style_name);
             if (isset($style_obj)) {
                 if ($style_obj->getProperty('rel-column-width') != NULL) {
diff --git a/ODT/styles/ODTParagraphStyle.php b/ODT/styles/ODTParagraphStyle.php
index a3f5697..1f4ece9 100644
--- a/ODT/styles/ODTParagraphStyle.php
+++ b/ODT/styles/ODTParagraphStyle.php
@@ -104,7 +104,7 @@ public function __construct() {
      */
     public function importProperties($properties, $disabled=array()) {
         foreach ($properties as $property => $value) {
-            if ($disabled [$property] == 0) {
+            if (isset($disabled[$property]) && $disabled[$property] == 0) {
                 $this->setProperty($property, $value);
             }
         }
@@ -175,7 +175,7 @@ public function setProperty($property, $value) {
     public function getProperty($property) {
         $style_fields = ODTStyleStyle::getStyleProperties ();
         if (array_key_exists ($property, $style_fields)) {
-            return $this->style_properties [$property]['value'];
+            return isset($this->style_properties[$property]['value']) ? $this->style_properties[$property]['value'] : null;
         }
         $paragraph_fields = self::$paragraph_fields;
         if (array_key_exists ($property, $paragraph_fields)) {
@@ -333,20 +333,20 @@ public function toString() {
      */
     public static function createParagraphStyle(array $properties, array $disabled_props = NULL, ODTDocument $doc=NULL){
         // Convert 'text-decoration'.
-        if ( $properties ['text-decoration'] == 'line-through' ) {
+        if (isset($properties['text-decoration']) && $properties['text-decoration'] == 'line-through') {
             $properties ['text-line-through-style'] = 'solid';
         }
-        if ( $properties ['text-decoration'] == 'underline' ) {
+        if (isset($properties['text-decoration']) && $properties['text-decoration'] == 'underline') {
             $properties ['text-underline-style'] = 'solid';
         }
-        if ( $properties ['text-decoration'] == 'overline' ) {
+        if (isset($properties['text-decoration']) && $properties['text-decoration'] == 'overline') {
             $properties ['text-overline-style'] = 'solid';
         }
 
         // If the property 'vertical-align' has the value 'sub' or 'super'
         // then for ODT it needs to be converted to the corresponding 'text-position' property.
         // Replace sub and super with text-position.
-        $valign = $properties ['vertical-align'];
+        $valign = isset($properties['vertical-align']) ? $properties['vertical-align'] : null;
         if (!empty($valign)) {
             if ( $valign == 'sub' ) {
                 $properties ['text-position'] = '-33% 100%';
@@ -358,8 +358,8 @@ public static function createParagraphStyle(array $properties, array $disabled_p
         }
 
         // Separate country from language
-        $lang = $properties ['lang'];
-        $country = $properties ['country'];
+        $lang = isset($properties['lang']) ? $properties['lang'] : null;
+        $country = isset($properties['country']) ? $properties['country'] : null;
         if ( !empty($lang) ) {
             $parts = preg_split ('/-/', $lang);
             $lang = $parts [0];
@@ -391,10 +391,10 @@ public static function createParagraphStyle(array $properties, array $disabled_p
         }
 
         // Eventually create parent for font-size
-        $save = $disabled_props ['font-size'];
+        $save = isset($disabled_props['font-size']) ? $disabled_props['font-size'] : null;
         $odt_fo_size = '';
         if ( empty ($disabled_props ['font-size']) ) {
-            $odt_fo_size = $properties ['font-size'];
+            $odt_fo_size = isset($properties['font-size']) ? $properties['font-size'] : null;
         }
         $parent = '';
         $length = strlen ($odt_fo_size);
@@ -411,7 +411,10 @@ public static function createParagraphStyle(array $properties, array $disabled_p
         }
 
         // Create style name (if not given).
-        $style_name = $properties ['style-name'];
+        $style_name = null;
+        if (array_key_exists('style-name', $properties)) {
+            $style_name = $properties ['style-name'] ?? null;
+        }
         if ( empty($style_name) ) {
             $style_name = self::getNewStylename ('Paragraph');
             $properties ['style-name'] = $style_name;
diff --git a/ODT/styles/ODTStyle.php b/ODT/styles/ODTStyle.php
index e76cf16..6d295f7 100644
--- a/ODT/styles/ODTStyle.php
+++ b/ODT/styles/ODTStyle.php
@@ -75,7 +75,7 @@ abstract public function setProperty($property, $value);
      * @return string The current value of the property
      */
     public function getProperty($property) {
-        return (isset($this->properties [$property]['value']) ? $this->properties [$property]['value'] : NULL);
+        return $this->properties[$property]['value'] ?? null;
     }
 
     /**
@@ -217,7 +217,7 @@ protected function importODTStyleInternal(array $fields, $xmlCode, &$properties=
      */
     protected function importPropertiesInternal(array $fields, $properties, $disabled, &$dest=NULL) {
         foreach ($properties as $property => $value) {
-            if ($disabled [$property] == 0 && array_key_exists ($property, $fields)) {
+            if (isset($disabled[$property]) && $disabled[$property] == 0 && array_key_exists($property, $fields)) {
                 $this->setPropertyInternal($property, $fields[$property][0], $value, $fields[$property][1], $dest);
             }
         }
diff --git a/ODT/styles/ODTTableCellStyle.php b/ODT/styles/ODTTableCellStyle.php
index b589c33..356e771 100644
--- a/ODT/styles/ODTTableCellStyle.php
+++ b/ODT/styles/ODTTableCellStyle.php
@@ -205,7 +205,10 @@ static public function getTableCellProperties () {
      */
     public static function createTableCellStyle(array $properties, array $disabled_props = NULL){
         // Create style name (if not given).
-        $style_name = $properties ['style-name'];
+        $style_name = null;
+        if (array_key_exists('style-name', $properties)) {
+            $style_name = $properties ['style-name'];
+        }
         if ( empty($style_name) ) {
             $style_name = self::getNewStylename ('TableCell');
             $properties ['style-name'] = $style_name;
diff --git a/ODT/styles/ODTTableColumnStyle.php b/ODT/styles/ODTTableColumnStyle.php
index cb3ef27..81699d9 100644
--- a/ODT/styles/ODTTableColumnStyle.php
+++ b/ODT/styles/ODTTableColumnStyle.php
@@ -149,7 +149,10 @@ public static function createTableColumnStyle(array $properties, array $disabled
         }
 
         // Convert width to ODT format
-        $table_co_width = $properties ['width'];
+        $table_co_width = null;
+        if (array_key_exists('width', $properties)) {
+            $table_co_width = $properties ['width'] ?? null;
+        }
         if ( !empty ($table_co_width) ) {
             $length = strlen ($table_co_width);
             if ( $table_co_width [$length-1] != '%' ) {
diff --git a/ODT/styles/ODTTableStyle.php b/ODT/styles/ODTTableStyle.php
index 258ba61..ba3333a 100644
--- a/ODT/styles/ODTTableStyle.php
+++ b/ODT/styles/ODTTableStyle.php
@@ -178,10 +178,10 @@ public static function createTableTableStyle(array $properties, array $disabled_
         if (empty($properties ['align'])) {
             $properties ['align'] = 'center';
         }
-        if ($properties ['margin-left'] == '0') {
+        if (isset($properties['margin-left']) && $properties['margin-left'] == '0') {
             unset($properties ['margin-left']);
         }
-        if ($properties ['margin-right'] == '0') {
+        if (isset($properties['margin-right']) && $properties['margin-right'] == '0') {
             unset($properties ['margin-right']);
         }
 
@@ -206,7 +206,10 @@ public static function createTableTableStyle(array $properties, array $disabled_
         }
 
         // Create style name (if not given).
-        $style_name = $properties ['style-name'];
+        $style_name = null;
+        if (array_key_exists('style-name', $properties)) {
+            $style_name = $properties['style-name'];
+        }
         if ( empty($style_name) ) {
             $style_name = self::getNewStylename ('Table');
             $properties ['style-name'] = $style_name;
diff --git a/ODT/styles/ODTTextStyle.php b/ODT/styles/ODTTextStyle.php
index d9c20a0..c13a91d 100644
--- a/ODT/styles/ODTTextStyle.php
+++ b/ODT/styles/ODTTextStyle.php
@@ -232,20 +232,20 @@ static public function getTextProperties () {
      */
     public static function createTextStyle(array $properties, array $disabled_props = NULL, ODTDocument $doc=NULL){
         // Convert 'text-decoration'.
-        if ( $properties ['text-decoration'] == 'line-through' ) {
+        if (isset($properties['text-decoration']) && $properties['text-decoration'] == 'line-through') {
             $properties ['text-line-through-style'] = 'solid';
         }
-        if ( $properties ['text-decoration'] == 'underline' ) {
+        if (isset($properties['text-decoration']) && $properties['text-decoration'] == 'underline') {
             $properties ['text-underline-style'] = 'solid';
         }
-        if ( $properties ['text-decoration'] == 'overline' ) {
+        if (isset($properties['text-decoration']) && $properties['text-decoration'] == 'overline') {
             $properties ['text-overline-style'] = 'solid';
         }
 
         // If the property 'vertical-align' has the value 'sub' or 'super'
         // then for ODT it needs to be converted to the corresponding 'text-position' property.
         // Replace sub and super with text-position.
-        $valign = $properties ['vertical-align'];
+        $valign = isset($properties['vertical-align']) ? $properties['vertical-align'] : '';
         if (!empty($valign)) {
             if ( $valign == 'sub' ) {
                 $properties ['text-position'] = '-33% 100%';
@@ -257,8 +257,8 @@ public static function createTextStyle(array $properties, array $disabled_props
         }
 
         // Separate country from language
-        $lang = $properties ['lang'];
-        $country = $properties ['country'];
+        $lang = isset($properties['lang']) ? $properties['lang'] : '';
+        $country = isset($properties['country']) ? $properties['country'] : '';
         if ( !empty($lang) ) {
             $parts = preg_split ('/-/', $lang);
             $lang = $parts [0];
@@ -276,10 +276,10 @@ public static function createTextStyle(array $properties, array $disabled_props
         }
 
         // Extra handling for font-size in '%'
-        $save = $disabled_props ['font-size'];
+        $save = isset($disabled_props['font-size']) ? $disabled_props['font-size'] : '';
         $odt_fo_size = '';
         if ( empty ($disabled_props ['font-size']) ) {
-            $odt_fo_size = $properties ['font-size'];
+            $odt_fo_size = $properties['font-size'] ?? null;
         }
         $length = strlen ($odt_fo_size);
         if ( $length > 0 && $odt_fo_size [$length-1] == '%' && isset($doc)) {
diff --git a/ODT/styleset.php b/ODT/styleset.php
index 3f4b4dd..2e8ee02 100644
--- a/ODT/styleset.php
+++ b/ODT/styleset.php
@@ -209,7 +209,7 @@ public function addStyleInternal(&$dest, &$dest_by_name, ODTStyle $new, $overwri
             // The key for a normal style is the name.
             $name = $new->getProperty('style-name');
 
-            if (!isset($dest_by_name [$name])) {
+            if (!isset($dest_by_name[$name]) || $dest_by_name[$name] === null) {
                 $dest [] = $new;
                 if (!empty($name)) {
                     $dest_by_name [$name] = $new;
diff --git a/helper/config.php b/helper/config.php
index 300f25c..3c709b4 100644
--- a/helper/config.php
+++ b/helper/config.php
@@ -558,7 +558,7 @@ protected function loadIntern(&$warning, $refresh) {
 
                 // Check DokuWiki global configuration.
                 $dw_name = $this->hasDWGlobalSetting ($name);
-                if (!$value && isset($conf[$dw_name])) {
+                if (!$value && isset($conf[$dw_name]) && $conf[$dw_name] !== '') {
                     $this->setParam ($name, $conf[$dw_name]);
                 }
                 
@@ -576,7 +576,7 @@ protected function loadIntern(&$warning, $refresh) {
                 // Check meta data in case syntax tags have written
                 // the config parameters to it.
                 unset($value);
-                if (isset($odt_meta[$name])) $value = $odt_meta[$name];
+                $value = $odt_meta[$name] ?? null;
                 if($this->isMetaSetting($name) && !empty($value)) {
                     $this->setParam ($name, $value);
                 }
diff --git a/helper/cssimport.php b/helper/cssimport.php
index 2fd013c..c433c30 100644
--- a/helper/cssimport.php
+++ b/helper/cssimport.php
@@ -448,7 +448,7 @@ protected function explodeBorderShorthand (&$decls) {
                 }
                 if ( $border_style_set === false ) {
                     if (!isset($values [$index])) $values [$index] = 'none';
-                    switch ($values [$index]) {
+                    switch ($values[$index] ?? null) {
                         case 'none':
                         case 'dotted':
                         case 'dashed':
@@ -477,8 +477,9 @@ protected function explodeBorderShorthand (&$decls) {
                 if ( $border_color_set === false ) {
                     if (!isset($values [$index])) $values [$index] = 'initial';
                     $decls [] = new css_declaration ('border-color', $values [$index]);
+                    $decls[] = new css_declaration('border-color', $values[$index] ?? null);
                     foreach ($border_sides as $border_side) {
-                        $decls [] = new css_declaration ($border_side.'-color', $values [$index]);
+                        $decls [] = new css_declaration( $border_side.'-color', $values[$index] ?? null);
                     }
 
                     // This is the last value.
@@ -486,7 +487,7 @@ protected function explodeBorderShorthand (&$decls) {
                 }
             }
             foreach ($border_sides as $border_side) {
-                $decls [] = new css_declaration ($border_side, $values [0].' '.$values [1].' '.$values [2]);
+                $decls [] = new css_declaration ($border_side, ($values[0] ?? null) . ' ' . ($values[1] ?? null) . ' ' . ($values[2] ?? null));
             }
         }
     }
@@ -897,7 +898,8 @@ protected function adjustLengthValuesBorder ($callback, $rule=NULL) {
                 if (!isset($values [2])) $values [2] = 'currentcolor'; // border-color
                 $width =
                     call_user_func($callback, $this->property, $values [0], CSSValueType::StrokeOrBorderWidth, $rule);
-                $this->value = $width . ' ' . $values [1] . ' ' . $values [2];
+                $this->value = $width . ' ' . ($values[1] ?? null) . ' ' . ($values[2] ?? null);
+
             break;
         }
     }
diff --git a/renderer/page.php b/renderer/page.php
index c2f9540..ad09210 100644
--- a/renderer/page.php
+++ b/renderer/page.php
@@ -945,19 +945,19 @@ function singlequoteclosing() {
 
     function apostrophe() {
         global $lang;
-        $text .= $lang['apostrophe'];
+        $text = $lang['apostrophe'];
         $this->document->addPlainText($text);
     }
 
     function doublequoteopening() {
         global $lang;
-        $text .= $lang['doublequoteopening'];
+        $text = $lang['doublequoteopening'];
         $this->document->addPlainText($text);
     }
 
     function doublequoteclosing() {
         global $lang;
-        $text .= $lang['doublequoteclosing'];
+        $text = $lang['doublequoteclosing'];
         $this->document->addPlainText($text);
     }
 
@@ -1333,7 +1333,7 @@ function internallink($id, $name = NULL, $returnonly = false) {
         $name = $this->_getLinkTitle($name, $default, $isImage, $id);
 
         // build the absolute URL (keeping a hash if any)
-        list($id,$hash) = explode('#',$id,2);
+        list($id, $hash) = array_pad(explode('#', $id, 2), 2, null); // array_pad() is used to add a null value to the array returned by explode() if it has fewer than two elements.
         $url = wl($id,'',true);
         if($hash) $url .='#'.$hash;