In 1.36 the default serialization format for ParserCache has been changed from PHP serialization to JSON serialization. The cache is still compatible with old PHP-serialized entries, so for the most part the change should be unnoticed. However in case some extensions are installed which write non-JSON-serializable data to ParserOutput::setExtensionData, the cache will break for some pages. Setting this to false makes ParserCache use PHP serialization format for writing new cache entries, and all the cache entries already written in JSON are discarded.