Ensure processors are valid callables

src/Monolog/Handler/AbstractHandler.php

@@ -73,6 +73,9 @@ abstract class AbstractHandler implements HandlerInterface
      */
     public function pushProcessor($callback)
     {
+        if (!is_callable($callback)) {
+            throw new \InvalidArgumentException('Processors must be valid callables (callback or object with an __invoke method), '.var_export($callback, true).' given');
+        }
         array_unshift($this->processors, $callback);
     }
 

src/Monolog/Logger.php

@@ -116,6 +116,9 @@ class Logger
      */
     public function pushProcessor($callback)
     {
+        if (!is_callable($callback)) {
+            throw new \InvalidArgumentException('Processors must be valid callables (callback or object with an __invoke method), '.var_export($callback, true).' given');
+        }
         array_unshift($this->processors, $callback);
     }