Fatal error: Uncaught Error: Undefined constant "MB_OVERLOAD_STRING" in /var/www/html/libs/sodium_compat-1.6.1/src/Core/Util.php:915 Stack trace: #0 /var/www/html/libs/sodium_compat-1.6.1/src/Core/Util.php(757): ParagonIE_Sodium_Core_Util::isMbStringOverride() #1 /var/www/html/libs/sodium_compat-1.6.1/src/Core/Poly1305.php(25): ParagonIE_Sodium_Core_Util::strlen() #2 /var/www/html/coap_server/lib/MQTTAuth.php(43): ParagonIE_Sodium_Core_Poly1305::onetimeauth() #3 /var/www/html/coap_server/coap_server.php(25): require_once('...') #4 {main} thrown in /var/www/html/libs/sodium_compat-1.6.1/src/Core/Util.php on line 915