Check for json dependency, fixes #378

src/Monolog/Formatter/NormalizerFormatter.php

@@ -30,6 +30,9 @@ class NormalizerFormatter implements FormatterInterface
     public function __construct($dateFormat = null)
     {
         $this->dateFormat = $dateFormat ?: static::SIMPLE_DATE;
+        if (!function_exists('json_encode')) {
+            throw new \RuntimeException('PHP\'s json extension is required to use Monolog\'s NormalizerFormatter');
+        }
     }
 
     /**