refactored the implementation of the NR handler and its test:

* the test now checks that if the extension is not loaded, an exception is thrown
* the test now mocks the new relic native functions
* moved some parameters as constants in the handler class

src/Monolog/Handler/NewRelicHandler.php

@@ -11,6 +11,8 @@
 
 namespace Monolog\Handler;
 
+use Monolog\Handler\MissingExtensionException;
+
 /**
  * Class to record a log on a NewRelic application
  *
@@ -18,17 +20,34 @@ namespace Monolog\Handler;
  */
 class NewRelicHandler extends AbstractProcessingHandler
 {    
+    const ERROR_MISSING_EXTENSION = "The NewRelic PHP extension is not installed on this system, therefore you can't use the NewRelicHandler";
+    const NEWRELIC_EXTENSION_NAME = 'newrelic';
+    
     /**
      * {@inheritdoc}
      */
     protected function write(array $record)
     {
-        if (extension_loaded('newrelic')) {
+        if ($this->isNewRelicEnabled()) {
             newrelic_notice_error($record['message']);
 
             foreach ($record['context'] as $key => $parameter) {
                 newrelic_add_custom_parameter($key, $parameter);
             }
+            
+            return;
         }
+        
+        throw new MissingExtensionException(self::ERROR_MISSING_EXTENSION);
+    }
+    
+    /**
+     * Checks whether the NewRelic extension is enabled in the system.
+     * 
+     * @return bool
+     */
+    protected function isNewRelicEnabled()
+    {
+        return (bool) extension_loaded(self::NEWRELIC_EXTENSION_NAME);
     }
 }