Fix use of undefined constant STDERR under certain conditions.

Error message:
    Use of undefined constant STDERR - assumed 'STDERR' (this will throw
    an Error in a future version of PHP)

Author: zachborboa

README.md

@@ -288,7 +288,7 @@ Curl::setXmlDecoder($mixed)
 Curl::success($callback)
 Curl::unsetHeader($key)
 Curl::unsetProxy()
-Curl::verbose($on = true, $output = STDERR)
+Curl::verbose($on = true, $output = 'STDERR')
 MultiCurl::__construct($base_url = null)
 MultiCurl::__destruct()
 MultiCurl::addCurl(Curl $curl)

src/Curl/Curl.php

@@ -1296,8 +1296,15 @@ public function removeHeader($key)
      * @param  bool $on
      * @param  resource $output
      */
-    public function verbose($on = true, $output = STDERR)
+    public function verbose($on = true, $output = 'STDERR')
     {
+        if ($output === 'STDERR') {
+            if (!defined('STDERR')) {
+                define('STDERR', fopen('php://stderr', 'wb'));
+            }
+            $output = STDERR;
+        }
+
         // Turn off CURLINFO_HEADER_OUT for verbose to work. This has the side
         // effect of causing Curl::requestHeaders to be empty.
         if ($on) {