code: public function getImplementedFunctionNames()
      {
          $returnValue = [];
          foreach (self::$phpSpreadsheetFunctions as $functionName => $function) {
              if ($this->isImplemented($functionName)) {
                  $returnValue[] = $functionName;
              }
          }
  
          return $returnValue;
      }
docs: Get a list of implemented Excel function names.
  
  @return array