Thread:Project:Support desk/Fatal error upgrading from 1.20.2 to 1.22.1

After the upgrade of MediaWiki following the regular upgrade procedure, I receive the following error:

Notice: Use of undefined constant JSON_HEX_TAG - assumed 'JSON_HEX_TAG' in /var/mediawiki-1.22.1/includes/json/FormatJson.php on line 166 Notice: Use of undefined constant JSON_HEX_AMP - assumed 'JSON_HEX_AMP' in /var/mediawiki-1.22.1/includes/json/FormatJson.php on line 166 Fatal error: Call to undefined function json_encode in /var/mediawiki-1.22.1/includes/json/FormatJson.php on line 167

My PHP version is: 5.5.3-1ubuntu2.1 (apache2handler) My MySQL version is: 5.5.34-0ubuntu0.13.10.1

MediaWiki runs on Ubuntu 13.10