diff --git a/src/authnet/AuthnetWebhook.php b/src/authnet/AuthnetWebhook.php
index 3ae20aa..e56c347 100644
--- a/src/authnet/AuthnetWebhook.php
+++ b/src/authnet/AuthnetWebhook.php
@@ -132,10 +132,13 @@ protected function getAllHeaders()
         if (function_exists('apache_request_headers')) {
             $headers = apache_request_headers();
         } else {
-            $headers = array_filter($_SERVER, function($key) {
-                return strpos($key, 'HTTP_') === 0;
-            }, ARRAY_FILTER_USE_KEY);
+            $headers = [];
+            foreach ($_SERVER as $key => $value) {
+                if (strpos($key, 'HTTP_') === 0) {
+                    $headers[str_replace('_', '-', substr($key, 5))] = $value;
+                }
+            }
         }
         return $headers;
     }
-}
\ No newline at end of file
+}