Represent simple strings that happen to be callable as strings (#1734)

src/Serializer/AbstractSerializer.php

@@ -288,6 +288,12 @@ protected function serializeCallable($callable): string
             throw new \InvalidArgumentException(sprintf('Expecting callable, got %s', \is_object($callable) ? \get_class($callable) : \gettype($callable)));
         }
 
+        // Prefer a string representation for simple strings that might just
+        // happen to be callable, such as "file".
+        if (\is_string($callable) && strpos($callable, '::') === false && strpos($callable, '_') === false) {
+            return $this->serializeString($callable);
+        }
+
         try {
             if (\is_array($callable)) {
                 $reflection = new \ReflectionMethod($callable[0], $callable[1]);