PHP Fatal error: Call to undefined function mb_ strlen( ) in / home/ cdpviewen/ public_ html/ function. Fatal error: undefined function mb_ strlen( ) FiretronP75 Junior Member. Posts: 6 Threads: 3 Joined: Jun. Fatal error: Call to undefined function mb_ strlen( ). zabbix- frontend- php package does not depend. I see such error messages in apache log: PHP Fatal error: Uncaught Error: Call to undefined function mb_ strlen( ).