Update upgrade.php to v18:
Release Notes: Some 5.4 functions have been implemented, and possibly a few bugs fixed. While gzdecode existed before, hex2bin, session_status, zlib_encode, zlib_decode, http_redirect, http_response_code, get_declared_traits (stub), and safe_unserialize are new. Various flags for json_encode/_decode like JSON_HEX_AMP have been added, or the newly (5.4) introduced JSON_PRETTY_PRINT, JSON_UNESCAPED_SLASHES, and JSON_UNESCAPED_UNICODE. A not-yet/non-standard JSON_PARSE_JAVASCRIPT has been implemented, as well as a basic json_last_error() function. It's a largely untested development release, however
