Fatal error: Uncaught Error: Undefined constant "MB_OVERLOAD_STRING" in /volume1/web/swwagro/libraries/vendor/joomla/string/src/phputf8/utf8.php on line 38

Error: Undefined constant "MB_OVERLOAD_STRING" in /volume1/web/swwagro/libraries/vendor/joomla/string/src/phputf8/utf8.php on line 38

Call Stack:
    0.0118     353088   1. {main}() /volume1/web/swwagro/index.php:0
    0.0127     354448   2. require_once('/volume1/web/swwagro/includes/framework.php') /volume1/web/swwagro/index.php:40
    0.0263     358352   3. require_once('/volume1/web/swwagro/libraries/cms.php') /volume1/web/swwagro/includes/framework.php:18
    0.0267     359560   4. require('/volume1/web/swwagro/libraries/vendor/autoload.php') /volume1/web/swwagro/libraries/cms.php:36
    0.0268     359560   5. ComposerAutoloaderInit205c915b9c7d3e718e7c95793ee67ffe::getLoader() /volume1/web/swwagro/libraries/vendor/autoload.php:7
    0.0272     360680   6. composerRequire205c915b9c7d3e718e7c95793ee67ffe($fileIdentifier = '2fb9d6f23c8e8faefc193a4cde0cab4f', $file = '/volume1/web/swwagro/libraries/vendor/composer/../joomla/string/src/phputf8/utf8.php') /volume1/web/swwagro/libraries/vendor/composer/autoload_real.php:56
    0.0275     361192   7. require('/volume1/web/swwagro/libraries/vendor/joomla/string/src/phputf8/utf8.php') /volume1/web/swwagro/libraries/vendor/composer/autoload_real.php:66