Backport PHPOffice#779

Author: chunfeilung

src/PhpWord/Element/Image.php

@@ -336,6 +336,10 @@ public function getImageStringData($base64 = false)
         // Read image binary data and convert to hex/base64 string
         if ($this->sourceType == self::SOURCE_GD) {
             $imageResource = call_user_func($this->imageCreateFunc, $actualSource);
+            if ($this->imageType === 'image/png') {
+                // PNG images need to preserve alpha channel information
+                imagesavealpha($imageResource, true);
+            }
             ob_start();
             call_user_func($this->imageFunc, $imageResource);
             $imageBinary = ob_get_contents();

src/PhpWord/Writer/AbstractWriter.php

@@ -360,6 +360,10 @@ protected function addFilesToPackage(ZipArchive $zip, $elements)
             // Retrive GD image content or get local media
             if (isset($element['isMemImage']) && $element['isMemImage']) {
                 $image = call_user_func($element['createFunction'], $element['source']);
+                if ($this->imageType === 'image/png') {
+                    // PNG images need to preserve alpha channel information
+                    imagesavealpha($imageResource, true);
+                }
                 ob_start();
                 call_user_func($element['imageFunction'], $image);
                 $imageContents = ob_get_contents();