Add get_meta_value

Some meta tag may use value instead of content property

Added `get_meta_value` to handle this kind of case, also when both content nor value property is present, will return null

Author: gavinclive

src/OpenGraph.php

@@ -31,11 +31,11 @@ public function fetch($url, $allMeta = null, $lang = null, $options = LIBXML_NOW
             $metaproperty = ($tag->hasAttribute('property')) ? $tag->getAttribute('property') : $tag->getAttribute('name');
             if (!$allMeta && $metaproperty && strpos($tag->getAttribute('property'), 'og:') === 0) {
                 $key = strtr(substr($metaproperty, 3), '-', '_');
-                $value = $tag->getAttribute('content');
+                $value = $this->get_meta_value($tag);
             }
             if ($allMeta && $metaproperty) {
                 $key = (strpos($metaproperty, 'og:') === 0) ? strtr(substr($metaproperty, 3), '-', '_') : $metaproperty;
-                $value = $tag->getAttribute('content');
+                $value = $this->get_meta_value($tag);
             }
             if (!empty($key)) {
                 $metadata[$key] = $value;
@@ -110,4 +110,17 @@ protected function verify_image_url($url)
             return false;
         }
     }
+
+    protected function get_meta_value($tag)
+    {
+        if (!empty($tag->getAttribute('content'))) {
+            $value = $tag->getAttribute('content');
+        } elseif (!empty($tag->getAttribute('value'))) {
+            $value = $tag->getAttribute('value');
+        } else {
+            $value = '';
+        }
+
+        return $value;
+    }
 }