] [ php7: error] [ pid 1856: tid 1572] [ client 188. 106: 61315] PHP Fatal error: Uncaught Error: Undefined class constant. Otherwise, returns _ escaped_ version of the value. PHP Fatal error: Uncaught PDOException: PDO: : _ _ construct( ) : Peer certificate. PHP message: Error: Undefined class constant. then your PHP version does not support that parameter and you need to upgrade. SOLVED] PHP fatal error ' Undefined class constant. is set to TRUE then the MySQL driver will use the buffered versions of the MySQL API. Since updating to PHP7 yesterday, I' m constantly seeing this error in the Apache error logs: [ Wed. 128: 2672] PHP Fatal error: Uncaught Error: Undefined class constant. We' re currently running XF version 1.