compile_string didn't return a tuple with shortcode dependencies.

nikola/plugins/compile/php.py

@@ -52,7 +52,7 @@ def compile(self, source, dest, is_two_file=True, post=None, lang=None):
 
     def compile_string(self, data, source_path=None, is_two_file=True, post=None, lang=None):
         """Compile PHP into HTML strings."""
-        return data
+        return data, []
 
     def create_post(self, path, **kw):
         """Create a new post."""