Fixes based on comments.

- Change function to private.
- Use private variable correctly.

src/Monolog/Handler/StreamHandler.php

@@ -61,7 +61,7 @@ class StreamHandler extends AbstractProcessingHandler
             if (!$this->url) {
                 throw new \LogicException('Missing stream url, the stream can not be opened. This may be caused by a premature call to close().');
             }
-            $errorMessage = null;
+            $this->errorMessage = null;
             set_error_handler(array($this, 'customErrorHandler'));
             $this->stream = fopen($this->url, 'a');
             restore_error_handler();
@@ -73,7 +73,7 @@ class StreamHandler extends AbstractProcessingHandler
         fwrite($this->stream, (string) $record['formatted']);
     }
 
-    protected function customErrorHandler($code, $msg) {
+    private function customErrorHandler($code, $msg) {
         $this->errorMessage = preg_replace('{^fopen\(.*?\): }', '', $msg);
     }
 }